RDMA/usnic: Annotate struct usnic_uiom_chunk 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 usnic_uiom_chunk.

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

Cc: Christian Benvenuti <benve@cisco.com>
Cc: Nelson Escobar <neescoba@cisco.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/20230929180431.3005464-3-keescook@chromium.orgSigned-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/usnic/usnic_uiom.h

@@ -77,7 +77,7 @@ struct usnic_uiom_reg {
 struct usnic_uiom_chunk {
 	struct list_head		list;
 	int				nents;
-	struct scatterlist		page_list[];
+	struct scatterlist		page_list[] __counted_by(nents);
 };
 
 struct usnic_uiom_pd *usnic_uiom_alloc_pd(struct device *dev);