net: Allow to show socket-specific information in /proc/[pid]/fdinfo/[fd]

This adds .show_fdinfo to socket_file_ops, so protocols will be able
to print their specific data in fdinfo.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>