media: pvrusb2: fix uaf in pvr2_context_set_notify

[Syzbot reported]
BUG: KASAN: slab-use-after-free in pvr2_context_set_notify+0x2c4/0x310 drivers/media/usb/pvrusb2/pvrusb2-context.c:35
Read of size 4 at addr ffff888113aeb0d8 by task kworker/1:1/26

CPU: 1 PID: 26 Comm: kworker/1:1 Not tainted 6.8.0-rc1-syzkaller-00046-gf1a27f081c1f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
Workqueue: usb_hub_wq hub_event
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xd9/0x1b0 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:377 [inline]
 print_report+0xc4/0x620 mm/kasan/report.c:488
 kasan_report+0xda/0x110 mm/kasan/report.c:601
 pvr2_context_set_notify+0x2c4/0x310 drivers/media/usb/pvrusb2/pvrusb2-context.c:35
 pvr2_context_notify drivers/media/usb/pvrusb2/pvrusb2-context.c:95 [inline]
 pvr2_context_disconnect+0x94/0xb0 drivers/media/usb/pvrusb2/pvrusb2-context.c:272

Freed by task 906:
kasan_save_stack+0x33/0x50 mm/kasan/common.c:47
kasan_save_track+0x14/0x30 mm/kasan/common.c:68
kasan_save_free_info+0x3f/0x60 mm/kasan/generic.c:640
poison_slab_object mm/kasan/common.c:241 [inline]
__kasan_slab_free+0x106/0x1b0 mm/kasan/common.c:257
kasan_slab_free include/linux/kasan.h:184 [inline]
slab_free_hook mm/slub.c:2121 [inline]
slab_free mm/slub.c:4299 [inline]
kfree+0x105/0x340 mm/slub.c:4409
pvr2_context_check drivers/media/usb/pvrusb2/pvrusb2-context.c:137 [inline]
pvr2_context_thread_func+0x69d/0x960 drivers/media/usb/pvrusb2/pvrusb2-context.c:158

[Analyze]
Task A set disconnect_flag = !0, which resulted in Task B's condition being met
and releasing mp, leading to this issue.

[Fix]
Place the disconnect_flag assignment operation after all code in pvr2_context_disconnect()
to avoid this issue.

Reported-and-tested-by:  <syzbot+ce750e124675d4599449@syzkaller.appspotmail.com>
Fixes: e5be15c6 ("V4L/DVB (7711): pvrusb2: Fix race on module unload")
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>