iflib: Add support for SIOCGIFDOWNREASON ioctl

This change adds native support for the SIOCGIFDOWNREASON ioctl in iflib.

When ifconfig issues SIOCGIFDOWNREASON, the request is now routed through a
new driver callback (IFDI_GET_DOWNREASON). iflib allocates the ifdownreason
structure, calls the driver to fill the down-reason message, and then
returns the data back to ifconfig for display.

Without this change, iflib-based drivers cannot implement link-down reason
reporting even if the hardware provides the information.

No functional change for existing drivers unless they implement the new
IFDI_GET_DOWNREASON method. Existing drivers continue to behave as before.

Reviewed by: gallatin, erj, kgalazka, ssaxena, #iflib
Differential Revision: https://reviews.freebsd.org/D54045
MFC After: 1 week

sys/net/ifdi_if.m

@@ -117,6 +117,12 @@ CODE {
 	{
 		return (false);
 	}
+
+	static int
+	null_get_downreason(if_ctx_t _ctx __unused, struct ifdownreason *_ifdr __unused)
+	{
+		return (ENOTSUP);
+	}
 };
 
 #
@@ -364,3 +370,8 @@ METHOD bool needs_restart {
 	if_ctx_t _ctx;
 	enum iflib_restart_event _event;
 } DEFAULT null_needs_restart;
+
+METHOD int get_downreason {
+	if_ctx_t _ctx;
+	struct ifdownreason *_ifdr;
+} DEFAULT null_get_downreason;

sys/net/iflib.c

@@ -4540,6 +4540,11 @@ iflib_if_ioctl(if_t ifp, u_long command, caddr_t data)
 		err = IFDI_PRIV_IOCTL(ctx, command, data);
 		CTX_UNLOCK(ctx);
 		break;
+	case SIOCGIFDOWNREASON:
+		CTX_LOCK(ctx);
+		err = IFDI_GET_DOWNREASON(ctx, (struct ifdownreason *)data);
+		CTX_UNLOCK(ctx);
+		break;
 	default:
 		err = ether_ioctl(ifp, command, data);
 		break;