CVE-2023-53625: drm/i915/gvt: fix vgpu debugfs clean in remove
In the Linux kernel, the following vulnerability has been resolved:

drm/i915/gvt: fix vgpu debugfs clean in remove

Check carefully on root debugfs available when destroying vgpu,
e.g in remove case drm minor's debugfs root might already be destroyed,
which led to kernel oops like below.

Console: switching to colour dummy device 80x25
i915 0000:00:02.0: MDEV: Unregistering
intel_vgpu_mdev b1338b2d-a709-4c23-b766-cc436c36cdf0: Removing from iommu group 14
BUG: kernel NULL pointer dereference, address: 0000000000000150
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP
CPU: 3 PID: 1046 Comm: driverctl Not tainted 6.1.0-rc2+ #6
Hardware name: HP HP ProDesk 600 G3 MT/829D, BIOS P02 Ver. 02.44 09/13/2022
RIP: 0010:__lock_acquire+0x5e2/0x1f90
Code: 87 ad 09 00 00 39 05 e1 1e cc 02 0f 82 f1 09 00 00 ba 01 00 00 00 48 83 c4 48 89 d0 5b 5d 41 5c 41 5d 41 5e 41 5f c3 45 31 ff <48> 81 3f 60 9e c2 b6 45 0f 45 f8 83 fe 01 0f 87 55 fa ff ff 89 f0
RSP: 0018:ffff9f770274f948 EFLAGS: 00010046
RAX: 0000000000000003 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000150
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
R10: ffff8895d1173300 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000150 R14: 0000000000000000 R15: 0000000000000000
FS:  00007fc9b2ba0740(0000) GS:ffff889cdfcc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000150 CR3: 000000010fd93005 CR4: 00000000003706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 lock_acquire+0xbf/0x2b0
 ? simple_recursive_removal+0xa5/0x2b0
 ? lock_release+0x13d/0x2d0
 down_write+0x2a/0xd0
 ? simple_recursive_removal+0xa5/0x2b0
 simple_recursive_removal+0xa5/0x2b0
 ? start_creating.part.0+0x110/0x110
 ? _raw_spin_unlock+0x29/0x40
 debugfs_remove+0x40/0x60
 intel_gvt_debugfs_remove_vgpu+0x15/0x30 [kvmgt]
 intel_gvt_destroy_vgpu+0x60/0x100 [kvmgt]
 intel_vgpu_release_dev+0xe/0x20 [kvmgt]
 device_release+0x30/0x80
 kobject_put+0x79/0x1b0
 device_release_driver_internal+0x1b8/0x230
 bus_remove_device+0xec/0x160
 device_del+0x189/0x400
 ? up_write+0x9c/0x1b0
 ? mdev_device_remove_common+0x60/0x60 [mdev]
 mdev_device_remove_common+0x22/0x60 [mdev]
 mdev_device_remove_cb+0x17/0x20 [mdev]
 device_for_each_child+0x56/0x80
 mdev_unregister_parent+0x5a/0x81 [mdev]
 intel_gvt_clean_device+0x2d/0xe0 [kvmgt]
 intel_gvt_driver_remove+0x2e/0xb0 [i915]
 i915_driver_remove+0xac/0x100 [i915]
 i915_pci_remove+0x1a/0x30 [i915]
 pci_device_remove+0x31/0xa0
 device_release_driver_internal+0x1b8/0x230
 unbind_store+0xd8/0x100
 kernfs_fop_write_iter+0x156/0x210
 vfs_write+0x236/0x4a0
 ksys_write+0x61/0xd0
 do_syscall_64+0x55/0x80
 ? find_held_lock+0x2b/0x80
 ? lock_release+0x13d/0x2d0
 ? up_read+0x17/0x20
 ? lock_is_held_type+0xe3/0x140
 ? asm_exc_page_fault+0x22/0x30
 ? lockdep_hardirqs_on+0x7d/0x100
 entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7fc9b2c9e0c4
Code: 15 71 7d 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 80 3d 3d 05 0e 00 00 74 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 48 83 ec 28 48 89 54 24 18 48
RSP: 002b:00007ffec29c81c8 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000000000000000d RCX: 00007fc9b2c9e0c4
RDX: 000000000000000d RSI: 0000559f8b5f48a0 RDI: 0000000000000001
RBP: 0000559f8b5f48a0 R08: 0000559f8b5f3540 R09: 00007fc9b2d76d30
R10: 0000000000000000 R11: 0000000000000202 R12: 000000000000000d
R13: 00007fc9b2d77780 R14: 000000000000000d R15: 00007fc9b2d72a00
 </TASK>
Modules linked in: sunrpc intel_rapl_msr intel_rapl_common intel_pmc_core_pltdrv intel_pmc_core intel_tcc_cooling x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel ee1004 igbvf rapl vfat fat intel_cstate intel_uncore pktcdvd i2c_i801 pcspkr wmi_bmof i2c_smbus acpi_pad vfio_pci vfio_pci_core vfio_virqfd zram fuse dm
---truncated---
CVSS v3.1: 5.5