mm/filemap: fix a typo in comment "unneccssary"->"unnecessary"

There is a typo in comment, fix it.
Signed-off-by: Ethon Paul <ethp@qq.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Link: http://lkml.kernel.org/r/20200411065141.15936-1-ethp@qq.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1256,7 +1256,7 @@ EXPORT_SYMBOL_GPL(add_page_wait_queue);
  * instead.
  *
  * The read of PG_waiters has to be after (or concurrently with) PG_locked
- * being cleared, but a memory barrier should be unneccssary since it is
+ * being cleared, but a memory barrier should be unnecessary since it is
  * in the same byte as PG_locked.
  */
 static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem)