inet_diag: Switch from _GETSOCK to IPPROTO_ numbers

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When dumping the AF_INET/AF_INET6 sockets user will also specify the protocol,
so prepare the protocol diag handlers to work with IPPROTO_ constants.

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