s390/vmcp: simplify vmcp_ioctl()

vmcp_ioctl() has many different return statements and duplicates a lot
of mutex_unlock() calls. Simplify this so that only one return
statement and one mutex_unlock() call is left.

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