mlxsw: spectrum_span: Annotate struct mlxsw_sp_span 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 mlxsw_sp_span.

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

Cc: Petr Machata <petrm@nvidia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20230929180746.3005922-5-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/spectrum_span.c

@@ -31,7 +31,7 @@ struct mlxsw_sp_span {
 	refcount_t policer_id_base_ref_count;
 	atomic_t active_entries_count;
 	int entries_count;
-	struct mlxsw_sp_span_entry entries[];
+	struct mlxsw_sp_span_entry entries[] __counted_by(entries_count);
 };
 
 struct mlxsw_sp_span_analyzed_port {