To prevent a module being removed whilst attached to a frontend, and
hence xenbus calling into potentially invalid text, take a reference on
the module before calling the probe() method (dropping it if unsuccessful)
and drop the reference after returning from the remove() method.
NOTE: This allows the ad-hoc reference counting in xen-netback to be
removed. This will be done in a subsequent patch.
Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>