mlxsw: Annotate struct mlxsw_linecards 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_linecards.

[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/20230929180746.3005922-1-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/core.h

@@ -624,7 +624,7 @@ struct mlxsw_linecards {
 	struct mlxsw_linecard_types_info *types_info;
 	struct list_head event_ops_list;
 	struct mutex event_ops_list_lock; /* Locks accesses to event ops list */
-	struct mlxsw_linecard linecards[];
+	struct mlxsw_linecard linecards[] __counted_by(count);
 };
 
 static inline struct mlxsw_linecard *