net: tulip: Annotate struct mediatable 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 mediatable.

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

Cc: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922172858.3822653-13-keescook@chromium.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/dec/tulip/tulip.h

@@ -381,7 +381,7 @@ struct mediatable {
 	unsigned has_reset:6;
 	u32 csr15dir;
 	u32 csr15val;		/* 21143 NWay setting. */
-	struct medialeaf mleaf[];
+	struct medialeaf mleaf[] __counted_by(leafcount);
 };