dmaengine: st_fdma: Annotate struct st_fdma_desc 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 st_fdma_desc.

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

Cc: Vinod Koul <[e-mail]>
Cc: Patrice Chotard <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: "Gustavo A. R. Silva" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/st_fdma.h

···
 	struct st_fdma_chan *fchan;
 	bool iscyclic;
 	unsigned int n_nodes;
-	struct st_fdma_sw_node node[];
+	struct st_fdma_sw_node node[] __counted_by(n_nodes);
 };
 
 enum st_fdma_type {