nfp: tls: Avoid -Wflex-array-member-not-at-end warnings

nfp: tls: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

So, in order to avoid ending up with flexible-array members in the
middle of other structs, we use the `struct_group_tagged()` helper
to separate the flexible array from the rest of the members in the
flexible structure. We then use the newly created tagged `struct
nfp_crypto_req_add_front_hdr` to replace the type of the objects
causing trouble in a couple of structures.

We also want to ensure that when new members need to be added to the
flexible structure, they are always included within the newly created
tagged struct. For this, we use `static_assert()`. This ensures that the
memory layout for both the flexible structure and the new tagged struct
is the same after any changes.

Lastly, use container_of() to retrieve a pointer to the flexible
structure and, through that, access the flexible-array member when
needed.

So, with these changes, fix the following warnings:

drivers/net/ethernet/netronome/nfp/nfd3/../crypto/fw.h:58:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/ethernet/netronome/nfp/nfd3/../crypto/fw.h:65:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/aSfDqouLFcA4h8JX@kspp
Signed-off-by: Kees Cook <kees@kernel.org>