rbd-mirror: permit R/W operations against non-primary image

rbd-mirror: permit R/W operations against non-primary image

With the non-primary feature bit is enabled, mask-out the read-only
feature bit that will be set in the refresh image state machine if
the image has that feature bit set. This will ensure that only the
rbd-mirror daemon will be able to modify a non-primary image.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>