librbd: enable/disable implicit non-primary feature bit

librbd: enable/disable implicit non-primary feature bit

When promoted to primary, disable the non-primary feature bit and
when demoted (or created non-primary), enable the non-primary feature
bit. This will prevent all non rbd-mirror RBD clients from modifying
the RBD image.

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