We don't even need Giant here. It isn't protecting anything internal

to geom, and nothing we call requires it to be held. It's left over
from a time when the latter wasn't the case. Retire it.

Reviewed in concept: scottl@

sys/geom/geom_ctl.c

@@ -68,7 +68,7 @@ static d_ioctl_t g_ctl_ioctl;
 
 static struct cdevsw g_ctl_cdevsw = {
 	.d_version =	D_VERSION,
-	.d_flags =	D_NEEDGIANT,
+	.d_flags =	0,
 	.d_ioctl =	g_ctl_ioctl,
 	.d_name =	"g_ctl",
 };