Convert a mtx_lock(&Giant) to a mtx_unlock(&Giant) in nfsrv_link() to

prevent leakage of Giant.  With INVARIANTS, this results in an
assertion failure following execution of the RPC.  Without INVARIANTS,
it could result in problems if the NFS server is killed causing nfsd
to return to user space holding Giant.

Feet provided by:	brueffer

sys/nfsserver/nfs_serv.c

@@ -2677,7 +2677,7 @@ nfsrv_link(struct nfsrv_descript *nfsd, struct nfssvc_sock *slp,
 			VOP_UNLOCK(dirp, 0, td);
 		}
 	}
-	mtx_lock(&Giant);	/* VFS */
+	mtx_unlock(&Giant);	/* VFS */
 	NFSD_LOCK();
 ereply:
 	nfsm_reply(NFSX_POSTOPATTR(v3) + NFSX_WCCDATA(v3));