btrfs-progs: check if there is required kernel send stream version

When kernel does not have the send stream version 2 patches,
the btrfs send with --stream-version 2 would fail with out
giving the details what is wrong. This patch will help to
identify correctly that required kernel patches are missing.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>