block/rnbd-srv: Fix use after free in rnbd_srv_sess_dev_force_close

KASAN detect following BUG:
[  778.215311] ==================================================================
[  778.216696] BUG: KASAN: use-after-free in rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.219037] Read of size 8 at addr ffff88b1d6516c28 by task tee/8842

[  778.220500] CPU: 37 PID: 8842 Comm: tee Kdump: loaded Not tainted 5.10.0-pserver #5.10.0-1+feature+linux+next+20201214.1025+0910d71
[  778.220529] Hardware name: Supermicro Super Server/X11DDW-L, BIOS 3.3 02/21/2020
[  778.220555] Call Trace:
[  778.220609]  dump_stack+0x99/0xcb
[  778.220667]  ? rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.220715]  print_address_description.constprop.7+0x1e/0x230
[  778.220750]  ? freeze_kernel_threads+0x73/0x73
[  778.220896]  ? rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.220932]  ? rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.220994]  kasan_report.cold.9+0x37/0x7c
[  778.221066]  ? kobject_put+0x80/0x270
[  778.221102]  ? rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.221184]  rnbd_srv_sess_dev_force_close+0x38/0x60 [rnbd_server]
[  778.221240]  rnbd_srv_dev_session_force_close_store+0x6a/0xc0 [rnbd_server]
[  778.221304]  ? sysfs_file_ops+0x90/0x90
[  778.221353]  kernfs_fop_write+0x141/0x240
[  778.221451]  vfs_write+0x142/0x4d0
[  778.221553]  ksys_write+0xc0/0x160
[  778.221602]  ? __ia32_sys_read+0x50/0x50
[  778.221684]  ? lockdep_hardirqs_on_prepare+0x13d/0x210
[  778.221718]  ? syscall_enter_from_user_mode+0x1c/0x50
[  778.221821]  do_syscall_64+0x33/0x40
[  778.221862]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  778.221896] RIP: 0033:0x7f4affdd9504
[  778.221928] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b3 0f 1f 80 00 00 00 00 48 8d 05 f9 61 0d 00 8b 00 85 c0 75 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 41 54 49 89 d4 55 48 89 f5 53
[  778.221956] RSP: 002b:00007fffebb36b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  778.222011] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f4affdd9504
[  778.222038] RDX: 0000000000000002 RSI: 00007fffebb36c50 RDI: 0000000000000003
[  778.222066] RBP: 00007fffebb36c50 R08: 0000556a151aa600 R09: 00007f4affeb1540
[  778.222094] R10: fffffffffffffc19 R11: 0000000000000246 R12: 0000556a151aa520
[  778.222121] R13: 0000000000000002 R14: 00007f4affea6760 R15: 0000000000000002

[  778.222764] Allocated by task 3212:
[  778.223285]  kasan_save_stack+0x19/0x40
[  778.223316]  __kasan_kmalloc.constprop.7+0xc1/0xd0
[  778.223347]  kmem_cache_alloc_trace+0x186/0x350
[  778.223382]  rnbd_srv_rdma_ev+0xf16/0x1690 [rnbd_server]
[  778.223422]  process_io_req+0x4d1/0x670 [rtrs_server]
[  778.223573]  __ib_process_cq+0x10a/0x350 [ib_core]
[  778.223709]  ib_cq_poll_work+0x31/0xb0 [ib_core]
[  778.223743]  process_one_work+0x521/0xa90
[  778.223773]  worker_thread+0x65/0x5b0
[  778.223802]  kthread+0x1f2/0x210
[  778.223833]  ret_from_fork+0x22/0x30

[  778.224296] Freed by task 8842:
[  778.224800]  kasan_save_stack+0x19/0x40
[  778.224829]  kasan_set_track+0x1c/0x30
[  778.224860]  kasan_set_free_info+0x1b/0x30
[  778.224889]  __kasan_slab_free+0x108/0x150
[  778.224919]  slab_free_freelist_hook+0x64/0x190
[  778.224947]  kfree+0xe2/0x650
[  778.224982]  rnbd_destroy_sess_dev+0x2fa/0x3b0 [rnbd_server]
[  778.225011]  kobject_put+0xda/0x270
[  778.225046]  rnbd_srv_sess_dev_force_close+0x30/0x60 [rnbd_server]
[  778.225081]  rnbd_srv_dev_session_force_close_store+0x6a/0xc0 [rnbd_server]
[  778.225111]  kernfs_fop_write+0x141/0x240
[  778.225140]  vfs_write+0x142/0x4d0
[  778.225169]  ksys_write+0xc0/0x160
[  778.225198]  do_syscall_64+0x33/0x40
[  778.225227]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

[  778.226506] The buggy address belongs to the object at ffff88b1d6516c00
                which belongs to the cache kmalloc-512 of size 512
[  778.227464] The buggy address is located 40 bytes inside of
                512-byte region [ffff88b1d6516c00, ffff88b1d6516e00)

The problem is in the sess_dev release function we call
rnbd_destroy_sess_dev, and could free the sess_dev already, but we still
set the keep_id in rnbd_srv_sess_dev_force_close, which lead to use
after free.

To fix it, move the keep_id before the sysfs removal, and cache the
rnbd_srv_session for lock accessing,

Fixes: 78699805

 ("block/rnbd-srv: close a mapped device from server side.")
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>