btrfs: stripe_alloc: hold a stripe of margin across a preallocation

btrfs: stripe_alloc: hold a stripe of margin across a preallocation

Admission now charges a preallocation the roundup of its bytes to whole
stripes, but that charge lives only for the check: between the
reservation and the allocation only the raw bytes sit in bytes_may_use.
fallocate() is quick, but eight fsstress processes preallocating at the
fill edge were quick together, and each in-flight preallocation short of
a stripe still took a stripe the gate had counted for someone else: the
failure-time dump kept showing admitted writes with the supply gone.

Do for preallocation what the delalloc margin does per outstanding
extent: hold one stripe in bytes_stripe_margin from the successful data
reservation until the caller's allocations are done, at the four users
of btrfs_alloc_data_chunk_ondemand() -- fallocate's range loop, the
zero-range preallocation, relocation's cluster preallocation and the
encoded write's extent reservation.  Released as soon as the claims have
landed, so the hold costs nothing beyond that window.

Assisted-by: Claude:claude-fable-5