net/smc: Use static_assert() to check struct sizes

Commit 9748dbc9 ("net/smc: Avoid -Wflex-array-member-not-at-end
warnings") introduced tagged `struct smc_clc_v2_extension_fixed` and
`struct smc_clc_smcd_v2_extension_fixed`. We want to ensure that when
new members need to be added to the flexible structures, they are
always included within these tagged structs.

So, we use `static_assert()` to ensure that the memory layout for
both the flexible structure and the tagged struct is the same after
any changes.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jan Karcher <jaka@linux.ibm.com>
Link: https://patch.msgid.link/ZrVBuiqFHAORpFxE@cute


Signed-off-by: Jakub Kicinski <kuba@kernel.org>