s390/kernel: squeeze a few more cycles out of the system call handler

Reorder the instructions of UPDATE_VTIME to improve superscalar execution,
remove duplicate checks for problem-state from the asynchronous interrupt
handlers, and move the check for problem-state from the synchronous
exit path to the program check path as it is only needed for program
checks inside the kernel.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>