net/smc: retain old name for diag_mode field

Commit c601171d ("net/smc: provide smc mode in smc_diag.c") changed
the name of diag_fallback field of struct smc_diag_msg structure
to diag_mode.  However, this structure is a part of UAPI, and this change
breaks user space applications that use it ([1], for example).  Since
the new name is more suitable, convert the field to a union that provides
access to the data via both the new and the old name.

[1] https://gitlab.com/strace/strace/blob/v4.24/netlink_smc_diag.c#L165

Fixes: c601171d ("net/smc: provide smc mode in smc_diag.c")
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/smc_diag.h

@@ -20,7 +20,10 @@ struct smc_diag_req {
 struct smc_diag_msg {
 	__u8		diag_family;
 	__u8		diag_state;
+	union {
 		__u8	diag_mode;
+		__u8	diag_fallback; /* the old name of the field */
+	};
 	__u8		diag_shutdown;
 	struct inet_diag_sockid id;