migration/rdma: unregister fd handler

migration/rdma: unregister fd handler

Unregister the fd handler before we destroy the channel,
otherwise we've got a race where we might land in the
fd handler just as we're closing the device.

(The race is quite data dependent, you just have to have
the right set of devices for it to trigger).

Corresponds to RH bz: https://bugzilla.redhat.com/show_bug.cgi?id=1666601

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190122173111.29821-1-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>