IB/hfi1: Annotate struct tid_rb_node 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 tid_rb_node.

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

Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.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/20230929180431.3005464-7-keescook@chromium.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>

Author: kees

drivers/infiniband/hw/hfi1/user_exp_rcv.h

@@ -36,7 +36,7 @@ struct tid_rb_node {
 	dma_addr_t dma_addr;
 	bool freed;
 	unsigned int npages;
-	struct page *pages[];
+	struct page *pages[] __counted_by(npages);
 };
 
 static inline int num_user_pages(unsigned long addr,