sched_ext: Fix the memleak for sch->helper objects

sched_ext: Fix the memleak for sch->helper objects

commit 517a44d18537ef8ab888f71197c80116c14cee0a upstream.

This commit use kthread_destroy_worker() to release sch->helper
objects to fix the following kmemleak:

unreferenced object 0xffff888121ec7b00 (size 128):
  comm "scx_simple", pid 1197, jiffies 4295884415
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 ad 4e ad de  .............N..
    ff ff ff ff 00 00 00 00 ff ff ff ff ff ff ff ff  ................
  backtrace (crc 587b3352):
    kmemleak_alloc+0x62/0xa0
    __kmalloc_cache_noprof+0x28d/0x3e0
    kthread_create_worker_on_node+0xd5/0x1f0
    scx_enable.isra.210+0x6c2/0x25b0
    bpf_scx_reg+0x12/0x20
    bpf_struct_ops_link_create+0x2c3/0x3b0
    __sys_bpf+0x3102/0x4b00
    __x64_sys_bpf+0x79/0xc0
    x64_sys_call+0x15d9/0x1dd0
    do_syscall_64+0xf0/0x470
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: bff3b5aec1b7 ("sched_ext: Move disable machinery into scx_sched")
Cc: stable@vger.kernel.org # v6.16+
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>