btrfs: send: factor out common logic when sending xattrs

We always send xattrs for the current inode only and both callers of
send_set_xattr() pass a path for the current inode. So move the path
allocation and computation to send_set_xattr(), reducing duplicated
code. This also facilitates an upcoming patch.

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