kernel: allow fork with TIF_NOTIFY_SIGNAL pending

fork() fails if signal_pending() is true, but there are two conditions
that can lead to that:

1) An actual signal is pending. We want fork to fail for that one, like
   we always have.

2) TIF_NOTIFY_SIGNAL is pending, because the task has pending task_work.
   We don't need to make it fail for that case.

Allow fork() to proceed if just task_work is pending, by changing the
signal_pending() check to task_sigpending().

Signed-off-by: Jens Axboe <axboe@kernel.dk>