rbd: disallow read-write partitions on images mapped read-only

rbd: disallow read-write partitions on images mapped read-only

If an image is mapped read-only, don't allow setting its partition(s)
to read-write via BLKROSET: with the previous patch all writes to such
images are failed anyway.

If an image is mapped read-write, its partition(s) can be set to
read-only (and back to read-write) as before.  Note that at the rbd
level the image will remain writeable: anything sent down by the block
layer will be executed, including any write from internal kernel users.

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