mm: fix some broken comments

Fix some broken comments including typo, grammar error and wrong function
name.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200913095456.54873-1-linmiaohe@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>