ifnet API: Change if_init() to take context argument

Some drivers, like iflib drivers, take a 'context' argument instead of a
ifnet argument, as a single interface may have multiple contexts.
Follow this scheme by passing the context argument down.  Most drivers
will likely pass 'ifp' as the context.

Reviewed by:	glebius
Sponsored by:	Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38102

sys/net/if.c

@@ -4622,9 +4622,9 @@ if_vlantrunkinuse(if_t ifp)
 }
 
 int
-if_init(if_t ifp)
+if_init(if_t ifp, void *ctx)
 {
-	(*((struct ifnet *)ifp)->if_init)((struct ifnet *)ifp);
+	(*((struct ifnet *)ifp)->if_init)(ctx);
 	return (0);
 }
 

sys/net/if_var.h

@@ -784,7 +784,7 @@ void if_bpfmtap(if_t ifp, struct mbuf *m);
 void if_etherbpfmtap(if_t ifp, struct mbuf *m);
 void if_vlancap(if_t ifp);
 int if_transmit(if_t ifp, struct mbuf *m);
-int if_init(if_t ifp);
+int if_init(if_t ifp, void *ctx);
 
 /*
  * Traversing through interface address lists.