btrfs: zoned: activate block group on allocation

Activate a block group when trying to allocate an extent from it. We check
read-only case and no space left case before trying to activate a block
group not to consume the number of active zones uselessly.

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