sched_ext: initialize kit->cursor.flags

struct bpf_iter_scx_dsq *it maybe not initialized.
If we didn't call scx_bpf_dsq_move_set_vtime and scx_bpf_dsq_move_set_slice
before scx_bpf_dsq_move, it would cause unexpected behaviors:
1. Assign a huge slice into p->scx.slice
2. Assign a invalid vtime into p->scx.dsq_vtime

Signed-off-by: Henry Huang <henry.hj@antgroup.com>
Fixes: 6462dd53 ("sched_ext: Compact struct bpf_iter_scx_dsq_kern")
Cc: stable@vger.kernel.org # v6.12
Signed-off-by: Tejun Heo <tj@kernel.org>