sched_ext: Move dsq_hash into scx_sched

sched_ext: Move dsq_hash into scx_sched

User DSQs are going to become per scheduler instance. Move dsq_hash into
scx_sched. This shifts the code that assumes scx_root to be the only
scx_sched instance up the call stack but doesn't remove them yet.

v2: Add missing rcu_read_lock() in scx_bpf_destroy_dsq() as per Andrea.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>