btrfs: send: implement fs_path_add_path() using fs_path_add()

The helper fs_path_add_path() is basically a copy of fs_path_add() and it
can be made a wrapper around fs_path_add(). So do that and also make it
inline and constify its second argument.

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