btrfs: send: simplify return logic from send_encoded_extent()

The 'out' label is pointless as we don't have anything to cleanup anymore
(we used to have an inode to iput), so remove it and make error paths
directly return an error.

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