mm: fix some typos in mm directory

No functional change.

Link: http://lkml.kernel.org/r/20190118235123.27843-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 90c13cd..6d3290c 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1301,7 +1301,7 @@
 
 /*
  * If it is possible to have holes within a MAX_ORDER_NR_PAGES, then we
- * need to check pfn validility within that MAX_ORDER_NR_PAGES block.
+ * need to check pfn validity within that MAX_ORDER_NR_PAGES block.
  * pfn_valid_within() should be used in this case; we optimise this away
  * when we have no holes within a MAX_ORDER_NR_PAGES block.
  */