osd/ReplicatedPG: fix trim of in-flight hit_sets

We normally need to stat the hit_set to know how many bytes to adjust the
stats by.  If the hit_set was just written, we will get ENOENT.

Get the obc instead, which will either get the in-memory copy (because the
repop is still in flight) or load it off of disk.

Fixes: #8283
Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1760 from ceph/wip-8283

osd/ReplicatedPG: fix trim of in-flight hit_sets

Reviewed-by: Samuel Just <sam.just@inktank.com>