mm/mmap.c: use mmap_assert_write_locked() instead of open coding it

In case the lock is actually not held at this point.

Link: https://lkml.kernel.org/r/5827758.TJ1SttVevJ@mobilepool36.emlix.com


Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>