Fix the server side krpc so that the kernel nfsd threads terminate.

Occationally the kernel nfsd threads would not terminate when a SIGKILL
was posted for the kernel process (called nfsd (slave)). When this occurred,
the thread associated with the process (called "ismaster") had returned from
svc_run_internal() and was sleeping waiting for the other threads to terminate.
The other threads (created by kthread_start()) were still in svc_run_internal()
handling NFS RPCs.
The only way this could occur is for the "ismaster" thread to return from
svc_run_internal() without having called svc_exit().
There was only one place in the code where this could happen and this patch
stops that from happening.
Since the problem is intermittent, I cannot be sure if this has fixed the
problem, but I have not seen an occurrence of the problem with this patch
applied.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D16087

sys/rpc/svc.c

@@ -1185,7 +1185,8 @@ svc_run_internal(SVCGROUP *grp, bool_t ismaster)
 			/*
 			 * Enforce maxthreads count.
 			 */
-			if (grp->sg_threadcount > grp->sg_maxthreads)
+			if (!ismaster && grp->sg_threadcount >
+			    grp->sg_maxthreads)
 				break;
 
 			/*