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

module/os/linux/spl/spl-thread.c

@@ -186,6 +186,13 @@ issig(void)
 
 		schedule();
 #endif
+		/*
+		 * Dequeued SIGSTOP/SIGTSTP.
+		 * Check if process has other singal pending.
+		 */
+		if (signal_pending(current))
+			return (1);
+
 		return (0);
 	}