osd/scrub: delay both targets on some failures

osd/scrub: delay both targets on some failures

If the failure of a scrub-job is due to a condition that affects
both targets, both should be delayed. Otherwise, we may end up
with the following bogus scenario:

A high priority deep target is scheduled, but scrub session initiation
fails due to, for example, a concurrent snap trim. The deep target
will be delayed. A second initiation attempt may happen after the
snap trimming is done, but before the updated deep target not-before.
As a result - the lower priority target will be scheduled before the
higher priority one - which is a bug.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>