extract_v4_addr(): Don't return a structure.

When compiling with -Waggregate-return, gcc issues the following warning:

	net.c: In function 'extract_v4_addr':
	net.c:682: warning: function returns an aggregate
	acl.c: In function 'acl_check_access':
	acl.c:60: warning: function call has aggregate value

It's not entirely clear how bad it is these days to return a structure, but
extract_v4_addr() is the only function in the tree which does it. Since it
can be easily changed to take a pointer instead and there is only a single
caller, let's implement it this way.