btrfs: zoned: prepare for allowing DUP on zoned

Allow for a block-group to be placed on more than one physical zone.

This is a preparation for allowing DUP profiles for meta-data on a zoned
file-system.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>