net: ipv6: Unify {raw,udp}6_sock_seq_show.

udp6_sock_seq_show and raw6_sock_seq_show are identical, except
the UDP version displays ports and the raw version displays the
protocol. Refactor most of the code in these two functions into
a new common ip6_dgram_sock_seq_show function, in preparation
for using it to display ICMPv6 sockets as well.

Also reduce the indentation in parts of include/net/transp_v6.h
to improve readability.

Compiles and displays reasonable results with CONFIG_IPV6={n,m,y}

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>