objectcacher: only want for commit

objectcacher: only want for commit

There was some old, weird stuff going on here where we would wait for the
ACK and COMMIT separately.  This is just wrong.  Writeback does not
complete until the data is committed on disk.

Simplify by waiting only for commit, removing all the 'ack' code, and
going back to a single callback (flush_set).

I didn't notice this for 05063867e2a54176ffc9bbc73391f52766ab403f; both of
these cleanups are needed to fix this.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>