s390/pkey: fix missing length of protected key on return

The pkey ioctl call PKEY_SEC2PROTK updates a struct pkey_protkey
on return. The protected key is stored in, the protected key type
is stored in but the len information was not updated. This patch
now fixes this and so the len field gets an update to refrect
the actual size of the protected key value returned.

Fixes: efc598e6 ("s390/zcrypt: move cca misc functions to new code file")
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reported-by: Christian Rund <RUNDC@de.ibm.com>
Suggested-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>