s390/traps: Copy monitor code to pt_regs

s390/traps: Copy monitor code to pt_regs

In case of a monitor call program check the CPU stores the monitor code to
lowcore. Let the program check handler copy it to the pt_regs structure so
it can be used by the monitor call exception handler.

Instead of increasing the pt_regs size add a union which contains both
orig_gpr2 and monitor_code, since orig_gpr2 is not used in case of a
program check.

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