osd/scrub: replace a ceph_assert() with a test

osd/scrub: replace a ceph_assert() with a test

We are using two distinct conditions to decide whether a candidate PG is already being scrubbed. The OSD checks pgs_scrub_active(), while the PG asserts on the value of PG_STATE_FLAG.
There is a time window when PG_STATE_FLAG is set but is_scrub_active() wasn't yet set. is_reserving() covers most of that period, but the ceph_assert is just before the is_reserving check.

fixes: https://tracker.ceph.com/issues/50346
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>