osd: transpose two wait lists in comment

osd: transpose two wait lists in comment

see PrimaryLogPG::do_request(), we check for

1. is_peered(), then
2. flushes_in_progress, then
3. is_active()
4. scrubber.is_chunky_scrub_active() && write_blocked_by_scrub(head) in
   PrimaryLogPG::do_op() which is called by PrimaryLogPG::do_request().

while in PrimaryLogPG::on_change()

we requeue the waiting request in the reversed order,

1. requeue_ops(waiting_for_peered);
2. requeue_ops(waiting_for_flush);
3. requeue_ops(waiting_for_active);

so the comment is not in sync with the code. in this change,
"waiting_for_active" and "waiting_for_flush" are transposed in the
comment explaining blocked request wait lists.

also, sync the pre-conditions of "waiting_for_peered" and
"waiting_for_flush" with "PrimaryLogPG::do_request()"

Signed-off-by: Kefu Chai <kchai@redhat.com>