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

(cherry picked from commit ef84dd8f4926304306d5989ca9afdbf760c6d813)
(cherry picked from commit 2f95e4a01e194428d65572ff3a3c97563120b38a)