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>