sparc: Annotate struct cpuinfo_tree with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct cpuinfo_tree.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175159.work.357-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

arch/sparc/kernel/cpumap.c

@@ -50,7 +50,7 @@ struct cpuinfo_tree {
 
 	/* Offsets into nodes[] for each level of the tree */
 	struct cpuinfo_level level[CPUINFO_LVL_MAX];
-	struct cpuinfo_node  nodes[];
+	struct cpuinfo_node  nodes[] __counted_by(total_nodes);
 };