Clear G_CF_ORPHAN when attaching. This fixes cases where the same
GEOM consumer can be orphaned, and then reattach to another provider.

From a user point of view, this makes gmountver(4) work again.

Reviewed by: avg, mav
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12228