uio: fix crash after the device is unregistered

uio: fix crash after the device is unregistered

For the target_core_user use case, after the device is unregistered
it maybe still opened in user space, then the kernel will crash, like:

[  251.163692] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[  251.163820] IP: [<ffffffffc0736213>] show_name+0x23/0x40 [uio]
[  251.163965] PGD 8000000062694067 PUD 62696067 PMD 0
[  251.164097] Oops: 0000 [#1] SMP
...
[  251.165605]  e1000 mptscsih mptbase drm_panel_orientation_quirks dm_mirror dm_region_hash dm_log dm_mod
[  251.166014] CPU: 0 PID: 13380 Comm: tcmu-runner Kdump: loaded Not tainted 3.10.0-916.el7.test.x86_64 #1
[  251.166381] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 05/19/2017
[  251.166747] task: ffff971eb91db0c0 ti: ffff971e9e384000 task.ti: ffff971e9e384000
[  251.167137] RIP: 0010:[<ffffffffc0736213>]  [<ffffffffc0736213>] show_name+0x23/0x40 [uio]
[  251.167563] RSP: 0018:ffff971e9e387dc8  EFLAGS: 00010282
[  251.167978] RAX: 0000000000000000 RBX: ffff971e9e3f8000 RCX: ffff971eb8368d98
[  251.168408] RDX: ffff971e9e3f8000 RSI: ffffffffc0738084 RDI: ffff971e9e3f8000
[  251.168856] RBP: ffff971e9e387dd0 R08: ffff971eb8bc0018 R09: 0000000000000000
[  251.169296] R10: 0000000000001000 R11: ffffffffa09d444d R12: ffffffffa1076e80
[  251.169750] R13: ffff971e9e387f18 R14: 0000000000000001 R15: ffff971e9cfb1c80
[  251.170213] FS:  00007ff37d175880(0000) GS:ffff971ebb600000(0000) knlGS:0000000000000000
[  251.170693] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  251.171248] CR2: 0000000000000008 CR3: 00000000001f6000 CR4: 00000000003607f0
[  251.172071] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  251.172640] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  251.173236] Call Trace:
[  251.173789]  [<ffffffffa0c9b2d3>] dev_attr_show+0x23/0x60
[  251.174356]  [<ffffffffa0f561b2>] ? mutex_lock+0x12/0x2f
[  251.174892]  [<ffffffffa0ac6d9f>] sysfs_kf_seq_show+0xcf/0x1f0
[  251.175433]  [<ffffffffa0ac54e6>] kernfs_seq_show+0x26/0x30
[  251.175981]  [<ffffffffa0a63be0>] seq_read+0x110/0x3f0
[  251.176609]  [<ffffffffa0ac5d45>] kernfs_fop_read+0xf5/0x160
[  251.177158]  [<ffffffffa0a3d3af>] vfs_read+0x9f/0x170
[  251.177707]  [<ffffffffa0a3e27f>] SyS_read+0x7f/0xf0
[  251.178268]  [<ffffffffa0f648af>] system_call_fastpath+0x1c/0x21
[  251.178823] Code: 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 53 48 89 d3 e8 7e 96 56 e0 48 8b 80 d8 02 00 00 48 89 df 48 c7 c6 84 80 73 c0 <48> 8b 50 08 31 c0 e8 e2 67 44 e0 5b 48 98 5d c3 0f 1f 00 66 2e
[  251.180115] RIP  [<ffffffffc0736213>] show_name+0x23/0x40 [uio]
[  251.180820]  RSP <ffff971e9e387dc8>
[  251.181473] CR2: 0000000000000008

CC: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
CC: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>