Btrfs: fix extent_from_logical to deal with skinny metadata

I don't think this is an issue and I've not seen it in practice but
extent_from_logical will fail to find a skinny extent because it uses
btrfs_previous_item and gives it the normal extent item type.  This is just not
a place to use btrfs_previous_item since we care about either normal extents or
skinny extents, so open code btrfs_previous_item to properly check.  This would
only affect metadata and the only place this is used for metadata is scrub and
I'm pretty sure it's just for printing stuff out, not actually doing any work so
hopefully it was never a problem other than a cosmetic one.  Thanks,
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/backref.c

@@ -1302,20 +1302,45 @@ int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
 	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
 	if (ret < 0)
 		return ret;
-	ret = btrfs_previous_item(fs_info->extent_root, path,
-					0, BTRFS_EXTENT_ITEM_KEY);
-	if (ret < 0)
-		return ret;
 
-	btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
+	while (1) {
+		u32 nritems;
+		if (path->slots[0] == 0) {
+			btrfs_set_path_blocking(path);
+			ret = btrfs_prev_leaf(fs_info->extent_root, path);
+			if (ret != 0) {
+				if (ret > 0) {
+					pr_debug("logical %llu is not within "
+						 "any extent\n", logical);
+					ret = -ENOENT;
+				}
+				return ret;
+			}
+		} else {
+			path->slots[0]--;
+		}
+		nritems = btrfs_header_nritems(path->nodes[0]);
+		if (nritems == 0) {
+			pr_debug("logical %llu is not within any extent\n",
+				 logical);
+			return -ENOENT;
+		}
+		if (path->slots[0] == nritems)
+			path->slots[0]--;
+
+		btrfs_item_key_to_cpu(path->nodes[0], found_key,
+				      path->slots[0]);
+		if (found_key->type == BTRFS_EXTENT_ITEM_KEY ||
+		    found_key->type == BTRFS_METADATA_ITEM_KEY)
+			break;
+	}
+
 	if (found_key->type == BTRFS_METADATA_ITEM_KEY)
 		size = fs_info->extent_root->leafsize;
 	else if (found_key->type == BTRFS_EXTENT_ITEM_KEY)
 		size = found_key->offset;
 
-	if ((found_key->type != BTRFS_EXTENT_ITEM_KEY &&
-	     found_key->type != BTRFS_METADATA_ITEM_KEY) ||
-	    found_key->objectid > logical ||
+	if (found_key->objectid > logical ||
 	    found_key->objectid + size <= logical) {
 		pr_debug("logical %llu is not within any extent\n", logical);
 		return -ENOENT;