net/smc: original socket family in inet_sock_diag

Commit ed75986f ("net/smc: ipv6 support for smc_diag.c") changed the
value of the diag_family field. The idea was to indicate the family of
the IP address in the inet_diag_sockid field. But the change makes it
impossible to distinguish an inet_sock_diag response message from SMC
sock_diag response. This patch restores the original behaviour and sends
AF_SMC as value of the diag_family field.

Fixes: ed75986f ("net/smc: ipv6 support for smc_diag.c")
Reported-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>