sched_ext: Preserve rq tracking across local DSQ dispatch

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

sched_ext: Preserve rq tracking across local DSQ dispatch

dispatch_to_local_dsq() can run from scx_bpf_dsq_move_to_local() while
ops.dispatch() has recorded the current rq. Moving a task to a local DSQ
may switch to the source or destination rq before synchronously invoking
ops.dequeue() through the following path:

SCX_CALL_OP(dispatch, rq)
ops.dispatch()
scx_bpf_dsq_move_to_local()
scx_flush_dispatch_buf()
finish_dispatch()
dispatch_to_local_dsq()
scx_dispatch_enqueue()
local_dsq_post_enq()
call_task_dequeue()
SCX_CALL_OP_TASK(dequeue, locked_rq, ...)

The nested callback saves the recorded rq and restores it on return. If
the rq tracking does not follow the lock switch, update_locked_rq() can
trigger the following lockdep assertion while restoring an rq which is
no longer held:

WARNING: kernel/sched/sched.h:1641 at call_task_dequeue+0x160/0x170
Call Trace:
scx_dispatch_enqueue+0x2b0/0x460
dispatch_to_local_dsq+0x138/0x230
scx_flush_dispatch_buf+0x1af/0x220
scx_bpf_dsq_move_to_local___v2+0xe2/0x1c0
bpf__sched_ext_ops_dispatch+0x4b/0xa7
do_pick_task_scx+0x3b6/0x910
__pick_next_task+0x105/0x1f0
__schedule+0x3e7/0x1980

Introduce switch_rq_lock() to update the tracking state together with
each rq lock handoff. Use it in dispatch_to_local_dsq(),
move_remote_task_to_local_dsq() and the in-balance paths of
scx_dsq_move(), ensuring that scx_locked_rq() consistently refers to the
rq whose lock is actually held throughout the lock dance.