btrfs: stripe_alloc: hold the probe margin until the metadata reservation charges it
btrfs_check_data_free_space() admits a write for its bytes plus a
whole-stripe margin per extent, then releases the margin at once, on the
understanding that btrfs_delalloc_reserve_metadata() re-charges it into
bytes_stripe_margin moments later. Between the two nothing holds it:
at the fill edge, eight fsstress writers were admitted into each other's
released margins, and the stripes their claims then needed had already
gone to their neighbours. The failure-time dump showed exactly that
shape -- claimable 0, bytes_may_use holding only the failing write, one
stripe of margin per outstanding extent, nothing stranded -- and it
survived charging preallocation by whole stripes and holding a stripe
across it, because it was never preallocation's stripe.
Let the callers that go on to reserve metadata keep the probe's margin
in bytes_may_use and release it only after that reservation has charged
bytes_stripe_margin: the buffered write, page_mkwrite, the block
truncation, the direct IO path (carried in btrfs_dio_data across the
two functions), the log-tail carry and btrfs_delalloc_reserve_space().
The one caller that reserves no metadata, the v1 space cache write-out,
keeps the immediate release.
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