net/mlx5e: Avoid a hundred -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 this particular case, we create a new `struct mlx5e_umr_wqe_hdr`
to enclose the header part of flexible structure `struct mlx5e_umr_wqe`.
This is, all the members except the flexible arrays `inline_mtts`,
`inline_klms` and `inline_ksms` in the anonymous union. We then replace
the header part with `struct mlx5e_umr_wqe_hdr hdr;` in `struct
mlx5e_umr_wqe`, and change the type of the object currently causing
trouble `umr_wqe` from `struct mlx5e_umr_wqe` to `struct
mlx5e_umr_wqe_hdr` --this last bit gets rid of the flex-array-in-the-middle
part and avoid the warnings.

Also, no new members should be added to `struct mlx5e_umr_wqe`, instead
any new members must be included in the header structure `struct
mlx5e_umr_wqe_hdr`. To enforce this, we use `static_assert()`, ensuring
that the memory layout of both the flexible structure and the newly
created header struct remain consistent.

The next step is to refactor the rest of the related code accordingly,
which means adding a bunch of `hdr.` wherever needed.

Lastly, we use `container_of()` whenever we need to retrieve a pointer
to the flexible structure `struct mlx5e_umr_wqe`.

So, with these changes, fix 125 of the following warnings:

drivers/net/ethernet/mellanox/mlx5/core/en.h:664:48: 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>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://patch.msgid.link/Z76HzPW1dFTLOSSy@kspp


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