bridge: Restore ABI compatibility with 14.x

When new fields were added to struct ifbreq in 15.0, the decision was
made to not use the padding to preserve binary compatibility.  However
this causes some issues, including the inability for 14.x jails to use
bridges, and a failure to bring up networking when booting a 15 kernel
with a 14.x userland, e.g. during upgrade.

Restore the old size of struct ifbreq by eating some of the padding.
This only requires 4 bytes of padding; we have 28 bytes left, and in
the medium term this problem will be solved with a netlink interface,
so running out of padding should not be a concern.

Fixes:	65ed1a035c ("bridge: allow member interface vlan to be configured")
Fixes:	f94c370de6 ("bridge: Allow VLAN protocol to be configured")
Reviewed by:	zlei, kevans, flo
Differential Revision:	https://reviews.freebsd.org/D52028

UPDATING

@@ -27,6 +27,15 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 15.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20250823:
+	To restore bridge(4)'s ABI compatibility with ifconfig from 14.x or
+	earlier, the size of struct ifbreq has changed, so you must update
+	the kernel and /sbin/ifconfig at the same time or you will not be
+	able to add interfaces to bridge.  This is particularly important
+	if you require bridge for network access during boot, in which case
+	you should install the new kernel and the new /sbin/ifconfig prior
+	to rebooting.
+
 20250820:
 	The WITHOUT_GSSAPI src.conf knob has been removed.  This was already
 	a no-op for MIT Kerberos, so this only affects builds which set

sys/net/if_bridgevar.h

@@ -159,7 +159,7 @@ struct ifbreq {
 	uint32_t	ifbr_addrexceeded;	/* member if addr violations */
 	ether_vlanid_t	ifbr_pvid;		/* member if PVID */
 	uint16_t	ifbr_vlanproto;		/* member if VLAN protocol */
-	uint8_t		pad[32];
+	uint8_t		pad[28];
 };
 
 /* BRDGGIFFLAGS, BRDGSIFFLAGS */