mm: fix some spelling mistakes in comments

mainline inclusion
from mainline-master
commit 60f7c503
category: bugfix
bugzilla: 175270
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=60f7c503d971a731ee3c4f884a9f2e80d476730d

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Fix some spelling mistakes in comments:
	udpate ==> update
	succesful ==> successful
	exmaple ==> example
	unneccessary ==> unnecessary
	stoping ==> stopping
	uknown ==> unknown

Link: https://lkml.kernel.org/r/20201127011747.86005-1-shihaitao1@huawei.com


Signed-off-by: Haitao Shi <shihaitao1@huawei.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Ouyangdelong <ouyangdelong@huawei.com>
Signed-off-by: Nifujia <nifujia1@hisilicon.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>