btrfs: relocation: Remove is_cowonly_root()

This function is only used in read_fs_root(), which is just a wrapper of
btrfs_get_fs_root().

For all the mentioned essential roots except log root tree,
btrfs_get_fs_root() has its own quick path to grab them from fs_info
directly, thus no need for key.offset modification.

For subvolume trees, btrfs_get_fs_root() with key.offset == -1 is
completely fine.

For log trees and log root tree, it's impossible to hit them, as for
relocation all backrefs are fetched from commit root, which never
records log tree blocks.

Log tree blocks either get freed in regular transaction commit, or
replayed at mount time. At runtime we should never hit an backref for
log tree in extent tree.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -640,21 +640,6 @@ static struct btrfs_root *find_reloc_root(struct reloc_control *rc,
 	return btrfs_grab_root(root);
 }
 
-static int is_cowonly_root(u64 root_objectid)
-{
-	if (root_objectid == BTRFS_ROOT_TREE_OBJECTID ||
-	    root_objectid == BTRFS_EXTENT_TREE_OBJECTID ||
-	    root_objectid == BTRFS_CHUNK_TREE_OBJECTID ||
-	    root_objectid == BTRFS_DEV_TREE_OBJECTID ||
-	    root_objectid == BTRFS_TREE_LOG_OBJECTID ||
-	    root_objectid == BTRFS_CSUM_TREE_OBJECTID ||
-	    root_objectid == BTRFS_UUID_TREE_OBJECTID ||
-	    root_objectid == BTRFS_QUOTA_TREE_OBJECTID ||
-	    root_objectid == BTRFS_FREE_SPACE_TREE_OBJECTID)
-		return 1;
-	return 0;
-}
-
 static struct btrfs_root *read_fs_root(struct btrfs_fs_info *fs_info,
 					u64 root_objectid)
 {
@@ -662,10 +647,7 @@ static struct btrfs_root *read_fs_root(struct btrfs_fs_info *fs_info,
 
 	key.objectid = root_objectid;
 	key.type = BTRFS_ROOT_ITEM_KEY;
-	if (is_cowonly_root(root_objectid))
-		key.offset = 0;
-	else
-		key.offset = (u64)-1;
+	key.offset = (u64)-1;
 
 	return btrfs_get_fs_root(fs_info, &key, false);
 }