powerpc/mm: fix __find_linux_pte() on 32 bits with PMD leaf entries

powerpc/mm: fix __find_linux_pte() on 32 bits with PMD leaf entries

Building on 32 bits with pmd_leaf() not returning always false leads to
the following error:

  CC      arch/powerpc/mm/pgtable.o
arch/powerpc/mm/pgtable.c: In function '__find_linux_pte':
arch/powerpc/mm/pgtable.c:506:1: error: function may return address of local variable [-Werror=return-local-addr]
  506 | }
      | ^
arch/powerpc/mm/pgtable.c:394:15: note: declared here
  394 |         pud_t pud, *pudp;
      |               ^~~
arch/powerpc/mm/pgtable.c:394:15: note: declared here

This is due to pmd_offset() being a no-op in that case.

So rework it for powerpc/32 so that pXd_offset() are used on real
pointers and not on on-stack copies.

Behind fixing the problem, it also has the advantage of simplifying
__find_linux_pte() including the removal of stack frame:

After this patch:

	00000018 <__find_linux_pte>:
	  18:	2c 06 00 00 	cmpwi   r6,0
	  1c:	41 82 00 0c 	beq     28 <__find_linux_pte+0x10>
	  20:	39 20 00 00 	li      r9,0
	  24:	91 26 00 00 	stw     r9,0(r6)
	  28:	2f 85 00 00 	cmpwi   cr7,r5,0
	  2c:	41 9e 00 0c 	beq     cr7,38 <__find_linux_pte+0x20>
	  30:	39 20 00 00 	li      r9,0
	  34:	99 25 00 00 	stb     r9,0(r5)
	  38:	54 89 65 3a 	rlwinm  r9,r4,12,20,29
	  3c:	7c 63 48 2e 	lwzx    r3,r3,r9
	  40:	2f 83 00 00 	cmpwi   cr7,r3,0
	  44:	41 9e 00 30 	beq     cr7,74 <__find_linux_pte+0x5c>
	  48:	54 69 07 3a 	rlwinm  r9,r3,0,28,29
	  4c:	2f 89 00 0c 	cmpwi   cr7,r9,12
	  50:	54 63 00 26 	clrrwi  r3,r3,12
	  54:	54 84 b5 36 	rlwinm  r4,r4,22,20,27
	  58:	3c 63 c0 00 	addis   r3,r3,-16384
	  5c:	7c 63 22 14 	add     r3,r3,r4
	  60:	4c be 00 20 	bnelr+  cr7
	  64:	4d 82 00 20 	beqlr
	  68:	39 20 00 17 	li      r9,23
	  6c:	91 26 00 00 	stw     r9,0(r6)
	  70:	4e 80 00 20 	blr
	  74:	38 60 00 00 	li      r3,0
	  78:	4e 80 00 20 	blr

Before this patch:

	00000018 <__find_linux_pte>:
	  18:	2c 06 00 00 	cmpwi   r6,0
	  1c:	94 21 ff e0 	stwu    r1,-32(r1)
	  20:	41 82 00 0c 	beq     2c <__find_linux_pte+0x14>
	  24:	39 20 00 00 	li      r9,0
	  28:	91 26 00 00 	stw     r9,0(r6)
	  2c:	2f 85 00 00 	cmpwi   cr7,r5,0
	  30:	41 9e 00 0c 	beq     cr7,3c <__find_linux_pte+0x24>
	  34:	39 20 00 00 	li      r9,0
	  38:	99 25 00 00 	stb     r9,0(r5)
	  3c:	54 89 65 3a 	rlwinm  r9,r4,12,20,29
	  40:	7c 63 48 2e 	lwzx    r3,r3,r9
	  44:	54 69 07 3a 	rlwinm  r9,r3,0,28,29
	  48:	2f 89 00 0c 	cmpwi   cr7,r9,12
	  4c:	90 61 00 0c 	stw     r3,12(r1)
	  50:	41 9e 00 4c 	beq     cr7,9c <__find_linux_pte+0x84>
	  54:	80 61 00 0c 	lwz     r3,12(r1)
	  58:	54 69 07 3a 	rlwinm  r9,r3,0,28,29
	  5c:	2f 89 00 0c 	cmpwi   cr7,r9,12
	  60:	90 61 00 08 	stw     r3,8(r1)
	  64:	41 9e 00 38 	beq     cr7,9c <__find_linux_pte+0x84>
	  68:	80 61 00 08 	lwz     r3,8(r1)
	  6c:	2f 83 00 00 	cmpwi   cr7,r3,0
	  70:	41 9e 00 54 	beq     cr7,c4 <__find_linux_pte+0xac>
	  74:	54 69 07 3a 	rlwinm  r9,r3,0,28,29
	  78:	2f 89 00 0c 	cmpwi   cr7,r9,12
	  7c:	54 69 00 26 	clrrwi  r9,r3,12
	  80:	54 8a b5 36 	rlwinm  r10,r4,22,20,27
	  84:	3c 69 c0 00 	addis   r3,r9,-16384
	  88:	7c 63 52 14 	add     r3,r3,r10
	  8c:	54 84 93 be 	srwi    r4,r4,14
	  90:	41 9e 00 14 	beq     cr7,a4 <__find_linux_pte+0x8c>
	  94:	38 21 00 20 	addi    r1,r1,32
	  98:	4e 80 00 20 	blr
	  9c:	54 69 00 26 	clrrwi  r9,r3,12
	  a0:	54 84 93 be 	srwi    r4,r4,14
	  a4:	3c 69 c0 00 	addis   r3,r9,-16384
	  a8:	54 84 25 36 	rlwinm  r4,r4,4,20,27
	  ac:	7c 63 22 14 	add     r3,r3,r4
	  b0:	41 a2 ff e4 	beq     94 <__find_linux_pte+0x7c>
	  b4:	39 20 00 17 	li      r9,23
	  b8:	91 26 00 00 	stw     r9,0(r6)
	  bc:	38 21 00 20 	addi    r1,r1,32
	  c0:	4e 80 00 20 	blr
	  c4:	38 60 00 00 	li      r3,0
	  c8:	38 21 00 20 	addi    r1,r1,32
	  cc:	4e 80 00 20 	blr

Link: https://lkml.kernel.org/r/50a3cfbab5b11890a0da027de5cb011a9d47ba89.1719928057.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>