btrfs: add a write-hole invariant checker to the raid56 write path

btrfs: add a write-hole invariant checker to the raid56 write path

With stripe-exclusive allocation, a raid56 data stripe may only be
written while an open or draining stripe run covers it: after its run
retires and drains at a transaction commit, nothing may ever write to
it again, and a write outside any run means an allocation bypassed the
policy.  Both cases are the write hole about to happen.

Check the invariant (under CONFIG_BTRFS_DEBUG) for every raid56 write
operation, full-stripe and sub-stripe alike, at rmw_rbio() time.  This
turns every upstream violation -- a missed allocation path, a
retirement ordering bug, an accounting leak -- into a deterministic
WARN at the moment of the offending write, instead of silent damage
that needs a crash plus a device failure plus a scrub to observe.  The
bios gathered in an rbio have not reported their IO done yet, so their
runs cannot drain under the check: no false positives from completion
races.

Block groups that ever hosted relocation-class runs are skipped
(sticky, debug-only flag): relocation legitimately overwrites its
preallocated extents in place after their runs drain, and the write
path cannot tell those writes from violations.

Assisted-by: Claude:claude-fable-5