rbd: treat images mapped read-only seriously

Even though -o ro/-o read_only/--read-only options are very old, we
have never really treated them seriously (on par with snapshots).  As
a first step, fail writes to images mapped read-only just like we do
for snapshots.

We need this check in rbd because the block layer basically ignores
read-only setting, see commit a32e236e ("Partially revert "block:
fail op_is_write() requests to read-only partitions"").

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