btrfs: sysfs: export supported send stream version

This reports the latest send stream version supported by the kernel as
the feature in /sys/fs/btrfs/features/send_stream_version .

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>