s390/crypt: fix missing unlock in ctr_paes_crypt on error path

The ctr mode of protected key aes uses the ctrblk page if the
ctrblk_lock could be acquired. If the protected key has to be
reestablished and this operation fails the unlock for the
ctrblk_lock is missing. Add it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>