udp_diag: Implement the get_exact dumping functionality

Do the same as TCP does -- lookup a socket in the given udp_table,
check cookie, fill the reply message with existing inet socket dumping
helper and send one back.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>