mm/filemap.c: fix minor typo

Link: http://lkml.kernel.org/r/20190304155240.19215-1-ldufour@linux.ibm.com


Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>