s390: add separate program check exit path

System call and program check handler both use the system call exit
path when returning to previous context. However the program check
handler jumps right to the end of the system call exit path if the
previous context is kernel context.

This lead to the quite odd double disabling of interrupts in the
system call exit path introduced with commit ce9dfafe ("s390:
fix system call exit path").

To avoid that have a separate program check handler exit path if the
previous context is kernel context.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>