btrfs: zoned: move partially zone_unusable block groups to reclaim list

btrfs: zoned: move partially zone_unusable block groups to reclaim list

On zoned block devices, block groups accumulate zone_unusable space
(space between the write pointer and zone end that cannot be allocated
until the zone is reset). When a block group becomes mostly
zone_unusable but still contains some valid data and it gets added to the
unused_bgs list it can never be deleted because it's not actually empty.

The deletion code (btrfs_delete_unused_bgs) skips these block groups
due to the btrfs_is_block_group_used() check, leaving them on the
unused_bgs list indefinitely. This causes two problems:
1. The block groups are never reclaimed, permanently wasting space
2. Eventually leads to ENOSPC even though reclaimable space exists

Fix by detecting block groups where zone_unusable exceeds 50% of the
block group size. Move these to the reclaim_bgs list instead of
skipping them. This triggers btrfs_reclaim_bgs_work() which:
1. Marks the block group read-only
2. Relocates the remaining valid data via btrfs_relocate_chunk()
3. Removes the emptied block group
4. Resets the zones, converting zone_unusable back to usable space

The 50% threshold ensures we only reclaim block groups where most space
is unusable, making relocation worthwhile. Block groups with less
zone_unusable are left on unused_bgs to potentially become fully empty
through normal deletion.

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