Commit v4.19-rc1~140^2~285^2~4 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: c601171d7a60 ("net/smc: provide smc mode in smc_diag.c")
Signed-off-by: Eugene Syromiatnikov <esyr@xxxxxxxxxx>