rbd: ask for a weaker incompat mask for read-only mappings

For a read-only mapping, ask for a set of features that make the image
only unwritable rather than both unreadable and unwritable by a client
that doesn't understand them.  As of today, the difference between them
for krbd is journaling (JOURNALING) and live migration (MIGRATING).

get_features method supports read_only parameter since hammer, ceph.git
commit 6176ec5fde2a ("librbd: differentiate between R/O vs R/W RBD
features").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>