[S390] fix syscall restart handling.

If do_signal() gets called several times before returning to user space
and no signal is pending (e.g. cancelled by a debugger) syscall restart
handling could be done several times. This would change the user space
PSW to an address prior to the syscall instruction.
Fix this by making sure that syscall restart handling is only done once.

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