rados.h,ReplicatedPG: add CEPH_OSD_FLAG_ENFORCE_SNAPC and use on flush

We need to ensure that even with pool snaps, we use the snapc provided in order
to ensure that the clones are written back correctly.

Fixes: #7941
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 499adb1db1cd225c91acce31b5f48fad5145043b)