crimson/osd: defer snap trimming while scrubbing

crimson/osd: defer snap trimming while scrubbing

Classic OSD enforces mutual exclusion between scrubbing and snap
trimming via the WaitScrub state in the snap trim state machine.
Crimson was missing this, allowing both to run concurrently on the
same PG (visible as active+clean+scrubbing+deep+snaptrim), which
could prevent snap trimming from completing within the expected
timeout.

Defer snap trim initiation while PG_STATE_SCRUBBING is set, and
re-trigger it from notify_scrub_end() via kick_snap_trim().

This is a temporary fix until the full scrub scheduling code,
including is_scrub_queued_or_active(), is merged.

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