btrfs: stripe_alloc: count open stripe run remainders against data reservations

btrfs: stripe_alloc: count open stripe run remainders against data reservations

Close the remaining reservation-vs-allocation windows the commit-time
stripe_unusable rescan cannot see:

- bytes_stripe_open (new): the sum of open runs' unallocated
  remainders, maintained per block group under stripe_run_lock at
  every open/alloc/grow/close, synced into the space_info after each
  mutation, and counted in btrfs_space_info_used().  Claimed bytes are
  invisible to the free space cache and will either be allocated or
  become trapped tails at close, so reservations must not be admitted
  against them.

A closed run's returned tail is still only counted as trapped by the
commit-time rescan, so reservations can race the retire-to-rescan window
inside a commit and be admitted against freshly trapped tails.  The next
patch closes that window from the free space cache side, where every
returning range is seen and no call site can be missed.

Measured on the fill-to-ENOSPC rolling test before this change (with
only the bytes_stripe_unusable accounting): tree 'b' still lost 3.3%
(8532 blocks) and tree 'c' 30% (62371 blocks) to silent writeback
allocation failure as trapping compounded.

Assisted-by: Claude:claude-fable-5