'struct g_gate_ctl_create' fields.

While the behaviour was technically undefined on other architectures
as well, on the reporter's amd64 systems the uninitialized bytes the
kernel cares about were always zero so everything worked as expected.

PR:		197309, 199559
Submitted by:	ota@j.email.ne.jp, Fabian Keil
Reviewed by:	pjd
MFC after:	1 week