os/bluestore: implement BlueRocksEnv::AreFilesSame()

os/bluestore: implement BlueRocksEnv::AreFilesSame()

it is used by the "repair" feature to dedup the files to be searched for
MANIFEST-* files. the default implementation is the POSIX one, which
tries to look at the local fs, but we should be looking for the files in
the bluefs. in this very use case, wal and db do not share the same device,
so we can just compare the paths. actually, it should aways return
"false". as the files being compared are always "db" and "db.wal".

Fixes: http://tracker.ceph.com/issues/21842
Signed-off-by: Kefu Chai <kchai@redhat.com>