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

nfp: 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 is currently an object (`tl`), at the beginning of multiple
structures, that contains a flexible structure (`struct nfp_dump_tl`),
for example:

struct nfp_dumpspec_csr {
        struct nfp_dump_tl tl;

        ...

        __be32 register_width;  /* in bits */
};

So, in order to avoid ending up with flexible-array members in the
middle of multiple other structs, we use the `struct_group_tagged()`
helper to separate the flexible array from the rest of the members
in the flexible structure:

struct nfp_dump_tl {
	struct_group_tagged(nfp_dump_tl_hdr, hdr,

	... the rest of members

	);
        char data[];
};

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

struct nfp_dumpspec_csr {
        struct nfp_dump_tl_hdr tl;

	...

        __be32 register_width;  /* in bits */
};

Also, use `container_of()` whenever we need to retrieve a pointer to
the flexible structure, through which we can access the flexible
array if needed.

So, with these changes, fix 33 of the following warnings:
drivers/net/ethernet/netronome/nfp/nfp_net_debugdump.c:58:28: 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/nfp_net_debugdump.c:64:28: 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/nfp_net_debugdump.c:70:28: 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/nfp_net_debugdump.c:78:28: 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/nfp_net_debugdump.c:87:28: 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/nfp_net_debugdump.c:92:28: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://github.com/KSPP/linux/issues/202
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZgYWlkxdrrieDYIu@neat
Signed-off-by: Jakub Kicinski <kuba@kernel.org>