udp_diag: Implement the dump-all functionality

Do the same as TCP does -- iterate the given udp_table, filter
sockets with bytecode and dump sockets into reply message.

The same filtering as for TCP applies, though only some of the
state bits really matter.

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