sched_ext: bpf_iter_scx_dsq_new() should always initialize iterator

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: bpf_iter_scx_dsq_new() should always initialize iterator

BPF programs may call next() and destroy() on BPF iterators even after new()

returns an error value (e.g. bpf_for_each() macro ignores error returns from

new()). bpf_iter_scx_dsq_new() could leave the iterator in an uninitialized

state after an error return causing bpf_iter_scx_dsq_next() to dereference

garbage data. Make bpf_iter_scx_dsq_new() always clear $kit->dsq so that

next() and destroy() become noops.