Several device drivers call if_alloc() and then do further checks and
will cal if_free() in case of conflict, error, ..
if_free() however sets the VNET instance from the ifp->if_vnet which
was not yet initialized but would only in if_attach(). Fix this by
setting the curvnet from where we allocate the interface in if_alloc().
if_attach() will later overwrite this as needed. We do not set the home_vnet
early on as we only want to prevent the if_free() panic but not change any
of the other housekeeping, e.g., triggered through ifioctl()s.

Reviewed by: brooks
Approved by: re (gjb)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7010