block: annotate struct request_queue with __counted_by_ptr

The queue_hw_ctx field in struct request_queue is an array of pointers
to struct blk_mq_hw_ctx. The number of elements in this array is tracked
by the nr_hw_queues field.

The array is allocated in __blk_mq_realloc_hw_ctxs() using
kcalloc_node() with set->nr_hw_queues elements. q->nr_hw_queues is
subsequently updated to set->nr_hw_queues.

When growing the array, the new array is assigned to queue_hw_ctx before
nr_hw_queues is updated. This is safe because nr_hw_queues (the old
smaller count) is used for bounds checking, which is within the new
larger allocation.

When shrinking the array, nr_hw_queues is updated to the smaller value,
while queue_hw_ctx retains the larger allocation. This is also safe as
the count is within the allocation bounds.

Annotating queue_hw_ctx with __counted_by_ptr(nr_hw_queues) allows the
compiler (with kSAN) to verify that accesses to queue_hw_ctx are within
the valid range defined by nr_hw_queues.

This patch was generated by CodeMender and reviewed by Bill Wendling.
Tested by running blktests.

Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Bill Wendling <morbo@google.com>
[axboe: massage commit message]
Signed-off-by: Jens Axboe <axboe@kernel.dk>