mm: fix some spelling mistakes in comments

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>
diff --git a/mm/page_ext.c b/mm/page_ext.c
index 16b161f2..df6f74a 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -34,7 +34,7 @@
  *
  * The need callback is used to decide whether extended memory allocation is
  * needed or not. Sometimes users want to deactivate some features in this
- * boot and extra memory would be unneccessary. In this case, to avoid
+ * boot and extra memory would be unnecessary. In this case, to avoid
  * allocating huge chunk of memory, each clients represent their need of
  * extra memory through the need callback. If one of the need callbacks
  * returns true, it means that someone needs extra memory so that