bcachefs: Don't use bch_write_op->cl for delivering completions

bcachefs: Don't use bch_write_op->cl for delivering completions

We already had op->end_io as an alternative mechanism to op->cl.parent
for delivering write completions; this switches all code paths to using
op->end_io.

Two reasons:
 - op->end_io is more efficient, due to fewer atomic ops, this completes
   the conversion that was originally only done for the direct IO path.
 - We'll be restructing the write path to use a different mechanism for
   punting to process context, refactoring to not use op->cl will make
   that easier.

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