osd/ReplicatedPG: do not include hit_set objects in full calculation

If we have a low target and there are hit_set objects (which cannot be
evicted), we can get stuck in a full state and never get out of it.

Fixes: #7674
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1430 from ceph/wip-7674

osd/ReplicatedPG: do not include hit_set objects in full calculation

Reviewed-by: Samuel Just <sam.just@inktank.com>