- Assert that the bufobj matches in flushbuflists. I still haven't gotten

to root cause on exactly how this happens.
 - If the assert is disabled, we presently try to handle this case, but the
   BUF_UNLOCK was missing.  Thus, if this condition ever hit we would leak
   a buf lock.

Many thanks to Peter Holm for all his help in finding this bug.  He really
put more effort into it than I did.

sys/kern/vfs_subr.c

@@ -1040,7 +1040,10 @@ flushbuflist(bufv, flags, bo, slpflag, slptimeo)
 			BO_LOCK(bo);
 			return (error != ENOLCK ? error : EAGAIN);
 		}
+		KASSERT(bp->b_bufobj == bo,
+	            ("wrong b_bufobj %p should be %p", bp->b_bufobj, bo));
 		if (bp->b_bufobj != bo) {	/* XXX: necessary ? */
+			BUF_UNLOCK(bp);
 			BO_LOCK(bo);
 			return (EAGAIN);
 		}