btrfs: zoned: make zone finishing multi stripe capable

Currently finishing of a zone only works if the block group isn't
spanning more than one zone.

This limitation is purely artificial and can be easily expanded to block
groups being places across multiple zones.

This is a preparation for allowing DUP and later more complex block-group
profiles on zoned btrfs.

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