btrfs: raid56: do not call a covered stripe_meta group uncovered

btrfs: raid56: do not call a covered stripe_meta group uncovered

report_uncovered_rmw() predates stripe_meta: it treats every metadata
read-modify-write as a write to a group the allocator does not cover
and says so, rate limited, on each one.  With raid56 metadata under
stripe_alloc those writes are partial writes inside one transaction's
run -- the audit it calls first classifies them (meta_rmw_cur) and
warns on its own if one ever rewrites parity over a committed block --
so the message told users of the covered configuration that the write
hole applied to them on every commit:

  read-modify-write of full stripe 42467328: this block group is not
  covered by stripe_alloc, the raid56 write hole applies to it

Keep the counter and the audit, and keep the message for what it was
written for: system chunks and mixed groups.

Assisted-by: Claude:claude-fable-5-1