librbd: make rbd_read_from_replica_policy actually work

librbd: make rbd_read_from_replica_policy actually work

Commit d92be8b5856d ("librbd: Improved support for balanced and
localized reads") introduced read_flags, but used low-level Objecter
flags (CEPH_OSD_FLAG_*).  This is wrong because librbd sits on top of
librados, not Objecter.  "balance" ends up being translated into
IGNORE_REDIRECT and "localize" being ignored.

Fixes: https://tracker.ceph.com/issues/45798
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit fc3c0c4dca2800140bd66100770403b1346cf738)