ReplicatedPG::scan_range: an object can disappear between the list and the attr get

The first item in the range is often last_backfill, upon which writes
can be occuring.  It's trimmed off on the primary side anyway.

Fixes: 10150
Backport: dumpling, firefly, giant
Signed-off-by: Samuel Just <sjust@redhat.com>
Merge pull request #3156 from athanatos/wip-10150

ReplicatedPG::scan_range: an object can disappear between the list and t...

Reviewed-by: Sage Weil <sage@redhat.com>