bcachefs: Nocow write error path fix

bcachefs: Nocow write error path fix

The nocow write error path was iterating over pointers in an extent,
aftre we'd dropped btree locks - oops.

Fortunately we'd already stashed what we need in nocow_lock_bucket, so
use that instead.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>