mm/frame_vector.c: release a semaphore in 'get_vaddr_frames()'

A semaphore is acquired before this check, so we must release it before
leaving.

Link: http://lkml.kernel.org/r/20171211211009.4971-1-christophe.jaillet@wanadoo.fr
Fixes: b7f0554a

 ("mm: fail get_vaddr_frames() for filesystem-dax mappings")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>