[S390] gmap: always up mmap_sem properly

If gmap_unmap_segment figures that the segment was not mapped in the
first place, it need to up mmap_sem on exit.

Cc: <stable@kernel.org>
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>