mm: percpu: Include smp.h in alloc_tag.h

percpu.h depends on smp.h, but doesn't include it directly because of
circular header dependency issues; percpu.h is needed in a bunch of low
level headers.

This fixes a randconfig build error on mips:

  include/linux/alloc_tag.h: In function '__alloc_tag_ref_set':
  include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id' [-Werror=implicit-function-declaration]

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 24e44cc2

 ("mm: percpu: enable per-cpu allocation tagging")
Closes: https://lore.kernel.org/oe-kbuild-all/202405210052.DIrMXJNz-lkp@intel.com/
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>