btrfs: zoned: add zone reclaim flush state for DATA space_info

btrfs: zoned: add zone reclaim flush state for DATA space_info

On zoned block devices, DATA block groups can accumulate large amounts
of zone_unusable space (space between the write pointer and zone end).
When zone_unusable reaches high levels (e.g., 98% of total space), new
allocations fail with ENOSPC even though space could be reclaimed by
relocating data and resetting zones.

The existing flush states don't handle this scenario effectively - they
either try to free cached space (which doesn't exist for zone_unusable)
or reset empty zones (which doesn't help when zones contain valid data
mixed with zone_unusable space).

Add a new RECLAIM_ZONES flush state that triggers the block group
reclaim machinery. This state:
- Calls btrfs_reclaim_sweep() to identify reclaimable block groups
- Calls btrfs_reclaim_bgs() to queue reclaim work
- Waits for reclaim_bgs_work to complete via flush_work()
- Commits the transaction to finalize changes

The reclaim work (btrfs_reclaim_bgs_work) safely relocates valid data
from fragmented block groups to other locations before resetting zones,
converting zone_unusable space back into usable space.

Insert RECLAIM_ZONES before RESET_ZONES in data_flush_states so that
we attempt to reclaim partially-used block groups before falling back
to resetting completely empty ones.

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