btrfs: raid56: do not park sync writes into nocow runs

btrfs: raid56: do not park sync writes into nocow runs

An in-place overwrite of a nodatacow file under stripe_alloc parked
like any partial write, but with a waiter behind it and nothing worth
merging: in-place overwrites arrive one fsync at a time, and unlike
the datacow fsync path nothing kicks the parked rbio before the page
writeback wait, so every fsync ate the full sync park deadline.
Measured against stock nodatacow on the same rig, that deadline was
the bulk of a ~3.7ms per-fsync regression.  Skip parking for sync
writes whose stripe belongs to a NOCOW-class run; async writeback
still parks and merges there.

Assisted-by: Claude:claude-fable-5