[S390] extmem unbalanced spin_lock.

segment save will exit with a lock held if the passed segment doesn't
exist. Any subsequent call to segment_save will lead to a deadlock.
Fix this and give up the lock before returning.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>