Need to reset the buffer pointer to avoid reconsidering the same buffer

again (without this the rollback analysis was being lost). Should reduce
the write count for most workloads.

Submitted by:	Craig A Soules <soules+@andrew.cmu.edu>

sys/kern/vfs_bio.c

@@ -1907,6 +1907,7 @@ flushbufqueues(void)
 				TAILQ_INSERT_TAIL(&bufqueues[QUEUE_DIRTY],
 				    bp, b_freelist);
 				bp->b_flags |= B_DEFERRED;
+				bp = TAILQ_FIRST(&bufqueues[QUEUE_DIRTY]);
 				continue;
 			}
 			vfs_bio_awrite(bp);