osd/scrub: allow new scrubs while reserving

osd/scrub: allow new scrubs while reserving

allow new scrub session to be initiated by an OSD even while a PG is
in the process of reserving scrub resources.
The existing restriction made sense when the replica reservation process
was expected to succeed or fail within a few milliseconds. It makes less
sense now that the reservation process is queue-based (Reserver based)
and can take unlimited time (hours, days, ...) to complete.

Fixes: https://tracker.ceph.com/issues/66351
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>