btrfs: send add define for v2 buffer size

Add a define for the data buffer size (though the maximum size is not
limited by it) BTRFS_SEND_BUF_SIZE_V2 so it's more visible.

Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>