Move sockaddr_is_loopback() to before sk_address_is_local(), and define the

Move sockaddr_is_loopback() to before sk_address_is_local(), and define the
latter in terms of the former.  Also adjust the definition of
ipv4_is_loopback() to avoid using the non-standard inet_netof() and
IN_LOOPBACKNET, and move it next to its remaining uses.

[originally from svn r5215]