smb: client, common: Avoid multiple -Wflex-array-member-not-at-end warnings

smb: client, common: Avoid multiple -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()` helper to
separate the flexible arrays from the rest of the members in the
flexible structures. We then use the newly created tagged `struct
smb2_file_link_info_hdr` and `struct smb2_file_rename_info_hdr`
to replace the type of the objects causing trouble: `rename_info`
and `link_info` in `struct smb2_compound_vars`.

We also want to ensure that when new members need to be added to the
flexible structures, they are always included within the newly created
tagged structs. 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.

So, with these changes, fix 86 of the following warnings:

fs/smb/client/cifsglob.h:2335:36: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/smb/client/cifsglob.h:2334:38: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>