When unloading driver by "modprobe -r amdgpu", one NULL pointer
dereference bug occurs in ras debugfs releasing. The cause is the
duplicated debugfs_remove, as drm debugfs_root dir has been cleaned
up already by drm_minor_unregister.

BUG: kernel NULL pointer dereference, address: 00000000000000a0
PGD 0 P4D 0
Oops: 0002 [#1] SMP PTI
CPU: 11 PID: 1526 Comm: modprobe Tainted: G           OE     5.6.0-guchchen #1
Hardware name: System manufacturer System Product Name/TUF Z370-PLUS GAMING II, BIOS 0411 09/21/2018
RIP: 0010:down_write+0x15/0x40
Code: eb de e8 7e 17 72 ff cc cc cc cc cc cc cc cc cc cc cc cc cc cc 0f 1f 44 00 00 53 48 89 fb e8 92
d8 ff ff 31 c0 ba 01 00 00 00 <f0> 48 0f b1 13 75 0f 65 48 8b 04 25 c0 8b 01 00 48 89 43 08 5b c3
RSP: 0018:ffffb1590386fcd0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 00000000000000a0 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffffff85b2fcc2 RDI: 00000000000000a0
RBP: ffffb1590386fd30 R08: ffffffff85b2fcc2 R09: 000000000002b3c0
R10: ffff97a330618c40 R11: 00000000000005f6 R12: ffff97a3481beb40
R13: 00000000000000a0 R14: ffff97a3481beb40 R15: 0000000000000000
FS:  00007fb11a717540(0000) GS:ffff97a376cc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000a0 CR3: 00000004066d6006 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 simple_recursive_removal+0x63/0x370
 ? debugfs_remove+0x60/0x60
 debugfs_remove+0x40/0x60
 amdgpu_ras_fini+0x82/0x230 [amdgpu]
 ? __kernfs_remove.part.17+0x101/0x1f0
 ? kernfs_name_hash+0x12/0x80
 amdgpu_device_fini+0x1c0/0x580 [amdgpu]
 amdgpu_driver_unload_kms+0x3e/0x70 [amdgpu]
 amdgpu_pci_remove+0x36/0x60 [amdgpu]
 pci_device_remove+0x3b/0xb0
 device_release_driver_internal+0xe5/0x1c0
 driver_detach+0x46/0x90
 bus_remove_driver+0x58/0xd0
 pci_unregister_driver+0x29/0x90
 amdgpu_exit+0x11/0x25 [amdgpu]
 __x64_sys_delete_module+0x13d/0x210
 do_syscall_64+0x5f/0x250
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>