mm/migrate: fix NULL movable_ops if CONFIG_ZSMALLOC=m

mm/migrate: fix NULL movable_ops if CONFIG_ZSMALLOC=m

After commit 84caf98838a3e5f4bdb34 ("mm: stop storing migration_ops in
page->mapping") we get such an error message if CONFIG_ZSMALLOC=m:

 WARNING: CPU: 3 PID: 42 at mm/migrate.c:142 isolate_movable_ops_page+0xa8/0x1c0
 CPU: 3 UID: 0 PID: 42 Comm: kcompactd0 Not tainted 6.16.0-rc5+ #2133 PREEMPT
 pc 9000000000540bd8 ra 9000000000540b84 tp 9000000100420000 sp 9000000100423a60
 a0 9000000100193a80 a1 000000000000000c a2 000000000000001b a3 ffffffffffffffff
 a4 ffffffffffffffff a5 0000000000000267 a6 0000000000000000 a7 9000000100423ae0
 t0 00000000000000f1 t1 00000000000000f6 t2 0000000000000000 t3 0000000000000001
 t4 ffffff00010eb834 t5 0000000000000040 t6 900000010c89d380 t7 90000000023fcc70
 t8 0000000000000018 u0 0000000000000000 s9 ffffff00010eb800 s0 ffffff00010eb800
 s1 000000000000000c s2 0000000000043ae0 s3 0000800000000000 s4 900000000219cc40
 s5 0000000000000000 s6 ffffff00010eb800 s7 0000000000000001 s8 90000000025b4000
    ra: 9000000000540b84 isolate_movable_ops_page+0x54/0x1c0
   ERA: 9000000000540bd8 isolate_movable_ops_page+0xa8/0x1c0
  CRMD: 000000b0 (PLV0 -IE -DA +PG DACF=CC DACM=CC -WE)
  PRMD: 00000004 (PPLV0 +PIE -PWE)
  EUEN: 00000000 (-FPE -SXE -ASXE -BTE)
  ECFG: 00071c1d (LIE=0,2-4,10-12 VS=7)
 ESTAT: 000c0000 [BRK] (IS= ECode=12 EsubCode=0)
  PRID: 0014c010 (Loongson-64bit, Loongson-3A5000)
 CPU: 3 UID: 0 PID: 42 Comm: kcompactd0 Not tainted 6.16.0-rc5+ #2133 PREEMPT
 Stack : 90000000021fd000 0000000000000000 9000000000247720 9000000100420000
         90000001004236a0 90000001004236a8 0000000000000000 90000001004237e8
         90000001004237e0 90000001004237e0 9000000100423550 0000000000000001
         0000000000000001 90000001004236a8 725a84864a19e2d9 90000000023fcc58
         9000000100420000 90000000024c6848 9000000002416848 0000000000000001
         0000000000000000 000000000000000a 0000000007fe0000 ffffff00010eb800
         0000000000000000 90000000021fd000 0000000000000000 900000000205cf30
         000000000000008e 0000000000000009 ffffff00010eb800 0000000000000001
         90000000025b4000 0000000000000000 900000000024773c 00007ffff103d748
         00000000000000b0 0000000000000004 0000000000000000 0000000000071c1d
         ...
 Call Trace:
 [<900000000024773c>] show_stack+0x5c/0x190
 [<90000000002415e0>] dump_stack_lvl+0x70/0x9c
 [<90000000004abe6c>] isolate_migratepages_block+0x3bc/0x16e0
 [<90000000004af408>] compact_zone+0x558/0x1000
 [<90000000004b0068>] compact_node+0xa8/0x1e0
 [<90000000004b0aa4>] kcompactd+0x394/0x410
 [<90000000002b3c98>] kthread+0x128/0x140
 [<9000000001779148>] ret_from_kernel_thread+0x28/0xc0
 [<9000000000245528>] ret_from_kernel_thread_asm+0x10/0x88

The reason is that defined(CONFIG_ZSMALLOC) evaluates to 1 only when
CONFIG_ZSMALLOC=y, we should use IS_ENABLED(CONFIG_ZSMALLOC) instead.  But
when I use IS_ENABLED(CONFIG_ZSMALLOC), page_movable_ops() cannot access
zsmalloc_mops because zsmalloc_mops is in a module.

To solve this problem, we define a set_movable_ops() interface to register
and unregister offline_movable_ops / zsmalloc_movable_ops in mm/migrate.c,
and call them at mm/balloon_compaction.c & mm/zsmalloc.c.  Since
offline_movable_ops / zsmalloc_movable_ops are always accessible, all
#ifdef / #endif are removed in page_movable_ops().

Link: https://lkml.kernel.org/r/20250817151759.2525174-1-chenhuacai@loongson.cn
Fixes: 84caf98838a3 ("mm: stop storing migration_ops in page->mapping")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>