s390: clear pt_regs::flags on irq entry

The current irq entry code doesn't initialize pt_regs::flags. On exit to
user mode arch_do_signal_or_restart() tests whether PIF_SYSCALL is set,
which might yield wrong results.

Fix this by clearing pt_regs::flags in the entry.S irq handler
code.

Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Fixes: 56e62a73 ("s390: convert to generic entry")
Cc: <stable@vger.kernel.org> # 5.12
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>