socket: fix unused-function warning

When procfs is disabled, the fdinfo code causes a harmless
warning:

net/socket.c:1000:13: error: 'sock_show_fdinfo' defined but not used [-Werror=unused-function]
 static void sock_show_fdinfo(struct seq_file *m, struct file *f)

Move the function definition up so we can use a single #ifdef
around it.

Fixes: b4653342 ("net: Allow to show socket-specific information in /proc/[pid]/fdinfo/[fd]")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>