s390/signal: switch to using vdso for sigreturn and syscall restart

with generic entry, there's a bug when it comes to restarting of signals.
The failing sequence is:

a) a signal is coming in, and no handler is registered, so the lower
   part of arch_do_signal_or_restart() in arch/s390/kernel/signal.c
   sets PIF_SYSCALL_RESTART.

b) a second signal gets pending while the kernel is still in the exit
   loop, and for that one, a handler exists.

c) The first part of arch_do_signal_or_restart() is called. That part
   calls handle_signal(), which sets up stack + registers for handling
   the signal.

d) __do_syscall() in arch/s390/kernel/syscall.c checks for
   PIF_SYSCALL_RESTART right before leaving to userspace. If it is set,
   it restart's the syscall. However, the registers are already setup
   for handling a signal from c). The syscall is now restarted with the
   wrong arguments.

Change the code to:

- use vdso for syscall_restart() instead of PIF_SYSCALL_RESTART because
  we cannot rewind and go back to userspace on s390 because the system call
  number might be encoded in the svc instruction.
- for all other syscalls we rewind the PSW and return to userspace.

Cc: <stable@kernel.org> # v5.12+ d57778fe: s390/vdso: always enable vdso
Cc: <stable@kernel.org> # v5.12+ 686341f2: s390/vdso64: add sigreturn,rt_sigreturn and restart_syscall
Cc: <stable@kernel.org> # v5.12+ 43e1f76b: s390/vdso: rename VDSO64_LBASE to VDSO_LBASE
Cc: <stable@kernel.org> # v5.12+ 779df224: s390/vdso: add minimal compat vdso
Cc: <stable@kernel.org> # v5.12+
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>