kernel: add support for TIF_NOTIFY_SIGNAL

kernel: add support for TIF_NOTIFY_SIGNAL

This adds TIF_NOTIFY_SIGNAL handling in the generic code, which if set,
will return true if signal_pending() is used in a wait loop. That causes
an exit of the loop so that notify_signal tracehooks can be run. If the
wait loop is currently inside a system call, the system call is restarted
once task_work has been processed.

In preparation for only having arch_do_signal() handle syscall restarts
if _TIF_SIGPENDING isn't set, rename it to arch_do_signal_or_restart().
Pass in a boolean that tells the arch signal handler if it should attempt
to get a signal, or just process a potential syscall restart.

For !CONFIG_GENERIC_ENTRY archs, we add the TIF_NOTIFY_SIGNAL handling
to get_signal(). This is done to minimize the needed arch changes to
support this feature.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>