mm, memcg: fix some typos in memcontrol.c

There are some typos in comment, fix them.

s/responsiblity/responsibility
s/oflline/offline

Signed-off-by: Ethon Paul <ethp@qq.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200411064246.15781-1-ethp@qq.com


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