osd/scrub: restart snap trimming after a failed scrub

osd/scrub: restart snap trimming after a failed scrub

A followup to PR#45640.
In PR#45640 snap trimming was restarted (if blocked) after all
successful scrubs, and after most scrub failures. Still, a few
failure scenarios did not handle snaptrim restart correctly.

The current PR cleans up and fixes the interaction between
scrub initiation/termination (for whatever cause) and snap
trimming.

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