mm: vmalloc: fix sparc64 warning

mm: vmalloc: fix sparc64 warning

This fixes this warning from a sparc64 defconfig build:

In file included from /home/sfr/next/next/include/linux/wait.h:11,
                 from /home/sfr/next/next/include/linux/swait.h:8,
                 from /home/sfr/next/next/include/linux/completion.h:12,
                 from /home/sfr/next/next/include/linux/mm_types.h:14,
                 from /home/sfr/next/next/include/linux/uio.h:10,
                 from /home/sfr/next/next/include/linux/vmalloc.h:12,
                 from /home/sfr/next/next/include/asm-generic/io.h:994,
                 from /home/sfr/next/next/arch/sparc/include/asm/io.h:22,
                 from /home/sfr/next/next/arch/sparc/vdso/vclock_gettime.c:18:
/home/sfr/next/next/arch/sparc/include/asm/current.h:18:30: warning: call-clobbered register used for global register variable
   18 | register struct task_struct *current asm("g4");
      |                              ^~~~~~~

Link: https://lkml.kernel.org/r/20230320144721.663280c3@canb.auug.org.au
Fixes: 4e29dd9708cb ("mm: vmalloc: convert vread() to vread_iter()")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>