osd/ReplicatedPG: fix hit_set_setup() on_activate()

Didn't read that if carefully the first time around!

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #967 from ceph/wip-7026

osd/ReplicatedPG: fix hit_set_setup() on_activate()
Reviewed-by: Greg Farnum <greg@inktank.com>