rbd: don't acquire exclusive lock for read-only mappings

A read-only mapping should be usable with read-only OSD caps, so
neither the header lock nor the object map lock can be acquired.
Unfortunately, this means that images mapped read-only lose the
advantage of the object map.

Snapshots, however, can take advantage of the object map without
any exclusionary locks, so if the object map is desired, snapshot
the image and map the snapshot instead of the image.

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