RDMA/uverbs: Avoid -Wflex-array-member-not-at-end warnings

RDMA/uverbs: Avoid -Wflex-array-member-not-at-end warnings

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

There are currently a couple of objects (`alloc_head` and `bundle`) in
`struct bundle_priv` that contain a couple of flexible structures:

struct bundle_priv {
        /* Must be first */
        struct bundle_alloc_head alloc_head;

	...

        /*
         * Must be last. bundle ends in a flex array which overlaps
         * internal_buffer.
         */
        struct uverbs_attr_bundle bundle;
        u64 internal_buffer[32];
};

So, in order to avoid ending up with a couple of flexible-array members
in the middle of a struct, we use the `struct_group_tagged()` helper to
separate the flexible array from the rest of the members in the flexible
structures:

struct uverbs_attr_bundle {
        struct_group_tagged(uverbs_attr_bundle_hdr, hdr,
		... the rest of the members
        );
        struct uverbs_attr attrs[];
};

With the change described above, we now declare objects of the type of
the tagged struct without embedding flexible arrays in the middle of
another struct:

struct bundle_priv {
        /* Must be first */
        struct bundle_alloc_head_hdr alloc_head;

        ...

        struct uverbs_attr_bundle_hdr bundle;
        u64 internal_buffer[32];
};

We also use `container_of()` whenever we need to retrieve a pointer
to the flexible structures.

Notice that the `bundle_size` computed in `uapi_compute_bundle_size()`
remains the same.

So, with these changes, fix the following warnings:

drivers/infiniband/core/uverbs_ioctl.c:45:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
   45 |         struct bundle_alloc_head alloc_head;
      |                                  ^~~~~~~~~~
drivers/infiniband/core/uverbs_ioctl.c:67:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
   67 |         struct uverbs_attr_bundle bundle;
      |                                   ^~~~~~

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZeIgeZ5Sb0IZTOyt@neat
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>