btrfs: avoid unnecessary path allocation when replaying a dir item

There's no need to allocate 'fixup_path' at replay_one_dir_item(), as the
path passed as an argument is unused by the time link_to_fixup_dir() is
called (replay_one_name() releases the path before it returns).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>