s390/pkey: Split pkey_unlocked_ioctl function

Split the very huge ioctl handling function pkey_unlocked_ioctl()
into individual functions per each IOCTL command.

There is no change in functional code coming with this patch.
The work is a simple copy-and-paste with the goal to have
the functionality absolutely untouched.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>