When allocating a pbuf for the cluster write, do not sleep waiting

for the available pbuf when passed vnode is backing md(4). Other i/o
directed to the same md device might already hold pbufs, and then we
could deadlock since only our progress can free a pbuf needed for
wakeup.

Obtained from:	projects/vm6
Reminded and tested by:	pho
MFC after:	1 week

sys/kern/vfs_cluster.c

@@ -837,7 +837,9 @@ cluster_wbuild(struct vnode *vp, long size, daddr_t start_lbn, int len,
 		  (tbp->b_bcount != tbp->b_bufsize) ||
 		  (tbp->b_bcount != size) ||
 		  (len == 1) ||
-		  ((bp = getpbuf(&cluster_pbuf_freecnt)) == NULL)) {
+		  ((bp = (vp->v_vflag & VV_MD) != 0 ?
+		  trypbuf(&cluster_pbuf_freecnt) :
+		  getpbuf(&cluster_pbuf_freecnt)) == NULL)) {
 			totalwritten += tbp->b_bufsize;
 			bawrite(tbp);
 			++start_lbn;