if_bridge: clean up INET/INET6 handling

if_bridge: clean up INET/INET6 handling

The if_bridge contains several instances of:

	if (AF_INET code ...
	#ifdef INET6
	    AF_INET6 code ...
	#endif
	) {
		...

Clean this up by adding a couple of macros at the top of the file that
are conditionally defined based on whether INET and/or INET6 are enabled,
which makes the code more readable and easier to maintain.

No functional change intended.

Reviewed by:	zlei, markj
MFC after:	1 week
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1191