md/md-llbitmap: add CleanUnwritten state for RAID-5 proactive parity building

md/md-llbitmap: add CleanUnwritten state for RAID-5 proactive parity building

Add new states to the llbitmap state machine to support proactive XOR
parity building for RAID-5 arrays. This allows users to pre-build parity
data for unwritten regions before any user data is written.

New states added:
- BitNeedSyncUnwritten: Transitional state when proactive sync is triggered
  via sysfs on Unwritten regions.
- BitSyncingUnwritten: Proactive sync in progress for unwritten region.
- BitCleanUnwritten: XOR parity has been pre-built, but no user data
  written yet. When user writes to this region, it transitions to BitDirty.

New actions added:
- BitmapActionProactiveSync: Trigger for proactive XOR parity building.
- BitmapActionClearUnwritten: Convert CleanUnwritten/NeedSyncUnwritten/
  SyncingUnwritten states back to Unwritten before recovery starts.

State flows:
- Current (lazy): Unwritten -> (write) -> NeedSync -> (sync) -> Dirty -> Clean
- New (proactive): Unwritten -> (sysfs) -> NeedSyncUnwritten -> (sync) -> CleanUnwritten
- On write to CleanUnwritten: CleanUnwritten -> (write) -> Dirty -> Clean
- On disk replacement: CleanUnwritten regions are converted to Unwritten
  before recovery starts, so recovery only rebuilds regions with user data

A new sysfs interface is added at /sys/block/mdX/md/llbitmap/proactive_sync
(write-only) to trigger proactive sync. This only works for RAID-456 arrays.

Link: https://lore.kernel.org/linux-raid/20260323054644.3351791-3-yukuai@fnnas.com/
Signed-off-by: Yu Kuai <yukuai@fnnas.com>