btrfs: stripe_alloc: private per-inode stripe runs for log-active inodes

btrfs: stripe_alloc: private per-inode stripe runs for log-active inodes

Closing the open stripe run covering every logged extent at each log
commit made completed fsyncs crash-safe, but at a placement cost: an
fsync-heavy inode shares the open band runs with every other writer, so
each of its log commits closes a shared run, fragmenting concurrent
write streams and trapping the shared run's tail.

Give log-active inodes their own runs instead.  The first fsync of an
inode sets a sticky runtime flag (at btrfs_sync_file entry, before the
fsync flushes its own delalloc, so even that first fsync's allocations
are steered).  Datacow allocations for a flagged inode come from a
private LOG-class run: claimed like any run but never placed in the
shared band slots, owned by the inode, and found by owner lookup under
the block group's stripe_run_lock.  A per-inode hint seeds the
allocator's search with the run's location; the hint is advisory (a
stale hint costs a lookup miss, never a wrong run, since the owner
match is authoritative).  When no fully-free stripes remain for a
private run the allocation falls back to the shared runs, restoring
the previous placement with unchanged safety.

One inode's log commit now settles only its own stripes: other
writers' runs stay open and their streams stay contiguous, and the
only trapped tails are the fsyncing inode's own.  Private runs also
give each log-active inode exclusive stripes, which a later change
uses to copy live tails forward and reclaim them without touching
foreign extents.

The commit-time retirement closed runs by walking the band slots,
which private runs never occupy; walk the block group's run list
instead so every open run, slotted or private, is closed and drained
under invariant I2 (no open run survives a transaction commit).

Assisted-by: Claude:claude-fable-5