Add an assertion that fifo_open() doesn't race against other threads

while sleeping to allocate fifo state: due to using the vnode lock to
serialize access to a fifo during open, it shouldn't happen (tm).

MFC after:	3 days

sys/fs/fifofs/fifo_vnops.c

@@ -196,6 +196,8 @@ fifo_open(ap)
 		SOCKBUF_LOCK(&rso->so_rcv);
 		rso->so_rcv.sb_state |= SBS_CANTRCVMORE;
 		SOCKBUF_UNLOCK(&rso->so_rcv);
+		KASSERT(vp->v_fifoinfo == NULL,
+		    ("fifo_open: v_fifoinfo race"));
 		vp->v_fifoinfo = fip;
 	}