inet_diag: Split inet_diag_get_exact into parts

The 1st part locks the inet handler and the 2nd one dump the
inet connection sock.

In the next patches the 1st part will be generalized to call
the socket dumping routine indirectly (i.e. TCP/UDP/DCCP) and
the 2nd part will be used by TCP and DCCP handlers.

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