btrfs: zoned: fix conventional zone capacity calculation

btrfs: zoned: fix conventional zone capacity calculation

When a block group contains both conventional zone and sequential zone, the
capacity of the block group is wrongly set to the block group's full
length. The capacity should be calculated in btrfs_load_block_group_* using
the last allocation offset.

Fixes: 568220fa9657 ("btrfs: zoned: support RAID0/1/10 on top of raid stripe tree")
CC: stable@vger.kernel.org # v6.12+
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>