mm: fix some comment errors

Link: https://lkml.kernel.org/r/20211101040208.460810-1-fuqf0919@gmail.com


Signed-off-by: Quanfa Fu <fuqf0919@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>