mm/util.c: fix strndup_user() comment

The kerneldoc misdescribes strndup_user()'s return value.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Timur Tabi <timur@freescale.com>
Cc: Mihai Caraman <mihai.caraman@freescale.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>