btrfs: zoned: activate new block group

Activate new block group at btrfs_make_block_group(). We do not check the
return value. If failed, we can try again later at the actual extent
allocation phase.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>