btrfs: send: allow protocol version 3 with CONFIG_BTRFS_DEBUG

We haven't finalized send stream v3 yet, so gate the send stream version
behind CONFIG_BTRFS_DEBUG as we want some way to test it.

The original verity send did not check the protocol version, so add that
actual protection as well.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>