librbd: commit IO as safe when complete if writeback cache is disabled

librbd: commit IO as safe when complete if writeback cache is disabled

We do not need to flush IO to ensure its safe if the writeback cache is
disabled when performing a journal replay. Instead, immediately mark the
IO as safe and let the journal's periodic commit throttle handle updating
the position.

Fixes: http://tracker.ceph.com/issues/23516
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit a27269ab95e4cadcade2131367e40ccf1011316a)

Conflicts:
	src/test/rbd_mirror/test_ImageReplayer.cc: removed "md_config_t::get_val" usage