btrfs: stripe_alloc: track per-sector liveness and pad from the map

btrfs: stripe_alloc: track per-sector liveness and pad from the map

Padding used to ask where the run's allocation frontier was, which
cannot see an allocation abandoned below it -- a reservation released, an
allocation the finder discarded, an extent freed before its write was
ever issued.  Runs now carry a bit per sector, set at allocation and
cleared by the new btrfs_open_stripe_write_abandoned() report, and
padding fills exactly the sectors that are dead: this replaces
btrfs_stripe_run_pad_start() with btrfs_stripe_run_pad_mask().  The map
is sized once, with headroom for frontier growth, because the growth
path holds the run lock where nothing may sleep; a run that would
outgrow its map stops growing instead.  A closed run's final stripe
overhangs its shrunk end by construction, and those sectors -- the
returned tail, unclaimable while the stripe holds live data -- are dead
and paddable too.

The abandonment report is what closes a writeback ENOSPC race the
fsync-heavy near-full workload hits: an allocation undone without a
report left phantom inflight bytes and phantom-live sectors, and
reservations admitted against that state failed at writeback
(cow_file_range -28) where stock refuses the write() up front.  With
the report in place the same workload shows zero writeback failures at
every reservation margin setting, including margin disabled.

Claiming also learns an O(1) fast fail: the by-size index walk is
extracted into find_free_stripe_run(), stopping at the first entry whose
largest contiguous free run cannot hold a full stripe -- near-full, where
free space degenerates into many sub-stripe holes, this replaces a scan
of the whole tree on every allocation.

Moved out of the stripe_meta patch, which needs all of this for tree
blocks but introduced it tangled with the metadata machinery; data wants
it on its own.

Assisted-by: Claude:claude-fable-5