btrfs: zoned: fix block group alloc_offset calculation

alloc_offset is offset from the start of a block group and @offset is
actually an address in logical space. Thus, we need to consider
block_group->start when calculating them.

Fixes: 011b41bf ("btrfs: zoned: advance allocation pointer after tree log node")
CC: stable@vger.kernel.org # 5.12+
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>