Another NFS SIGSTOP related fix: Ignore thread suspend requests due to

SIGSTOP if stop signals are currently deferred.  This can occur if a
process is stopped via SIGSTOP while a thread is running or runnable
but before it has set TDF_SBDRY.

Tested by:	pho
Reviewed by:	kib
MFC after:	1 week

sys/kern/kern_thread.c

@@ -794,6 +794,17 @@ thread_suspend_check(int return_instead)
 		    (p->p_flag & P_SINGLE_BOUNDARY) && return_instead)
 			return (ERESTART);
 
+		/*
+		 * Ignore suspend requests for stop signals if they
+		 * are deferred.
+		 */
+		if (P_SHOULDSTOP(p) == P_STOPPED_SIG &&
+		    td->td_flags & TDF_SBDRY) {
+			KASSERT(return_instead,
+			    ("TDF_SBDRY set for unsafe thread_suspend_check"));
+			return (0);
+		}
+
 		/*
 		 * If the process is waiting for us to exit,
 		 * this thread should just suicide.