Btrfs: fix possible memory leak in replace_path()

In replace_path(), if read_tree_block() fails, we cannot return
directly, we should free some allocated memory otherwise memory
leak happens.

Similar to Wang's "Btrfs: fix possible memory leak in the
find_parent_nodes()" patch, the current commit fixes an issue that
is related to the "Btrfs: fix all callers of read_tree_block"
commit.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>