Btrfs: do a full search everytime in btrfs_search_old_slot

While running some snashot aware defrag tests I noticed I was panicing every
once and a while in key_search. This is because of the optimization that says
if we find a key at slot 0 it will be at slot 0 all the way down the rest of the
tree. This isn't the case for btrfs_search_old_slot since it will likely replay
changes to a buffer if something has changed since we took our sequence number.
So short circuit this optimization by setting prev_cmp to -1 every time we call
key_search so we will do our normal binary search. With this patch I am no
longer seeing the panics I was seeing before. Thanks,

Signed-off-by: Josef Bacik <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/ctree.c

···
 	int level;
 	int lowest_unlock = 1;
 	u8 lowest_level = 0;
-	int prev_cmp;
+	int prev_cmp = -1;
 
 	lowest_level = p->lowest_level;
 	WARN_ON(p->nodes[0] != NULL);
···
 	}
 
 again:
-	prev_cmp = -1;
 	b = get_old_root(root, time_seq);
 	level = btrfs_header_level(b);
 	p->locks[level] = BTRFS_READ_LOCK;
···
 		 */
 		btrfs_unlock_up_safe(p, level + 1);
 
+		/*
+		 * Since we can unwind eb's we want to do a real search every
+		 * time.
+		 */
+		prev_cmp = -1;
 		ret = key_search(b, key, level, &prev_cmp, &slot);
 
 		if (level != 0) {