rbd: make common options override krbd-specific options

rbd: make common options override krbd-specific options

ceph-csi has added support for passing custom map and unmap options via
mapOptions and unmapOptions storage class parameters.  However, it also
uses --read-only for implementing ROX (ReadOnlyMany) PVs.  If the user
supplies "mapOptions: rw", they will get around the intended read-only
restriction (at least on the block device).

ceph-csi could be patched to use "-o ro", but it actually makes sense
for common options to win over device type-specific equivalents.

Fixes: https://tracker.ceph.com/issues/47625
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>