RDMA/siw: Annotate struct siw_pbl 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 siw_pbl.

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

Cc: Bernard Metzler <bmt@zurich.ibm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230929180431.3005464-4-keescook@chromium.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>

Author: kees

drivers/infiniband/sw/siw/siw.h

@@ -137,7 +137,7 @@ struct siw_pble {
 struct siw_pbl {
 	unsigned int num_buf;
 	unsigned int max_buf;
-	struct siw_pble pbe[];
+	struct siw_pble pbe[] __counted_by(max_buf);
 };
 
 /*