drm/amdkfd: Fix cat debugfs hang_hws file causes system crash bug

drm/amdkfd: Fix cat debugfs hang_hws file causes system crash bug

Here is the system crash log:
[ 1272.884438] BUG: unable to handle kernel NULL pointer dereference at
(null)
[ 1272.884444] IP: [<          (null)>]           (null)
[ 1272.884447] PGD 825b09067 PUD 8267c8067 PMD 0
[ 1272.884452] Oops: 0010 [#1] SMP
[ 1272.884509] CPU: 13 PID: 3485 Comm: cat Kdump: loaded Tainted: G
[ 1272.884515] task: ffff9a38dbd4d140 ti: ffff9a37cd3b8000 task.ti:
ffff9a37cd3b8000
[ 1272.884517] RIP: 0010:[<0000000000000000>]  [<          (null)>]
(null)
[ 1272.884520] RSP: 0018:ffff9a37cd3bbe68  EFLAGS: 00010203
[ 1272.884522] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
0000000000014d5f
[ 1272.884524] RDX: fffffffffffffff4 RSI: 0000000000000001 RDI:
ffff9a38aca4d200
[ 1272.884526] RBP: ffff9a37cd3bbed0 R08: ffff9a38dcd5f1a0 R09:
ffff9a31ffc07300
[ 1272.884527] R10: ffff9a31ffc07300 R11: ffffffffaddd5e9d R12:
ffff9a38b4e0fb00
[ 1272.884529] R13: 0000000000000001 R14: ffff9a37cd3bbf18 R15:
ffff9a38aca4d200
[ 1272.884532] FS:  00007feccaa67740(0000) GS:ffff9a38dcd40000(0000)
knlGS:0000000000000000
[ 1272.884534] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1272.884536] CR2: 0000000000000000 CR3: 00000008267c0000 CR4:
00000000003407e0
[ 1272.884537] Call Trace:
[ 1272.884544]  [<ffffffffade68940>] ? seq_read+0x130/0x440
[ 1272.884548]  [<ffffffffade40f8f>] vfs_read+0x9f/0x170
[ 1272.884552]  [<ffffffffade41e4f>] SyS_read+0x7f/0xf0
[ 1272.884557]  [<ffffffffae374ddb>] system_call_fastpath+0x22/0x27
[ 1272.884558] Code:  Bad RIP value.
[ 1272.884562] RIP  [<          (null)>]           (null)
[ 1272.884564]  RSP <ffff9a37cd3bbe68>
[ 1272.884566] CR2: 0000000000000000

Signed-off-by: Qu Huang <jinsdb@126.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>