net/smc: provide smc mode in smc_diag.c

Rename field diag_fallback into diag_mode and set the smc mode of a
connection explicitly.

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>