osd/scrubber: simplify existing reservation case in handle_scrub_reserve_request

osd/scrubber: simplify existing reservation case in handle_scrub_reserve_request

First, the is_stale() case should be impossible.  This patch leaves an
error message rather than an assert.

Second, this patch removes the special handling for receiving a reservation
when m_remote_osd_resource is already populated.  The next patch will exploit
the fact that all calls to m_remote_osd_resource.reset() and advance_token()
occur together.

Signed-off-by: Samuel Just <sjust@redhat.com>