nfp: Use static_assert() to check struct sizes

Commit d88cabfd ("nfp: Avoid -Wflex-array-member-not-at-end
warnings") introduced tagged `struct nfp_dump_tl_hdr`. We want
to ensure that when new members need to be added to the flexible
structure, they are always included within this tagged struct.

So, we use `static_assert()` to ensure that the memory layout for
both the flexible structure and the tagged struct is the same after
any changes.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/ZrVB43Hen0H5WQFP@cuteSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/netronome/nfp/nfp_net_debugdump.c

@@ -41,6 +41,8 @@ struct nfp_dump_tl {
 	);
 	char data[];
 };
+static_assert(offsetof(struct nfp_dump_tl, data) == sizeof(struct nfp_dump_tl_hdr),
+	      "struct member likely outside of struct_group_tagged()");
 
 /* NFP CPP parameters */
 struct nfp_dumpspec_cpp_isl_id {