Fix issig() to check signal_pending after dequeue SIGSTOP/SIGTSTP

When process got SIGSTOP/SIGTSTP, issig() dequeue them and return 0.
But process could still have another signal pending after dequeue. So,
after dequeue, check and return 1, if signal_pending.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jitendra Patidar <jitendra.patidar@nutanix.com>
Closes #16464