iflib: allow clone detach if not yet init

If we hit an error during init, then we'll unwind our state and attempt
to detach the device -- don't block it.

This was discovered by creating a wg0 with missing parameters; said
failure ended up leaving this orphaned device in place and ended up
panicking the system upon enumeration of the dev.* sysctl space.

Reviewed by:	gallatin, markj
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29145

sys/net/iflib_clone.c

@@ -83,7 +83,8 @@ iflib_pseudo_detach(device_t dev)
 	if_ctx_t ctx;
 
 	ctx = device_get_softc(dev);
-	if ((iflib_get_flags(ctx) & IFC_IN_DETACH) == 0)
+	if ((iflib_get_flags(ctx) & (IFC_INIT_DONE | IFC_IN_DETACH)) ==
+	    IFC_INIT_DONE)
 		return (EBUSY);
 	return (0);
 }