rbd: don't establish watch for read-only mappings

With exclusive lock out of the way, watch is the only thing left that
prevents a read-only mapping from being used with read-only OSD caps.

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