KVM: s390: Reworked SIGP RESTART order

When SIGP RESTART detected an illegal CPU address, there is no need to
drop to userspace, we can return CC3 to the guest directly instead.
Also renamed __sigp_restart() to sigp_check_callable() (since this
is a better description of what the function is really doing) and
moved a string specific to RESTART to the calling place instead, so
that this function gets usable by other SIGP orders, too.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>