ocfs2: rebase copied fsdlm LVB pointers in locking_state

In the Linux kernel, the following vulnerability has been resolved:

ocfs2: rebase copied fsdlm LVB pointers in locking_state

The locking_state debugfs iterator snapshots struct ocfs2_lock_res by
value under ocfs2_dlm_tracking_lock and later formats that copy in
ocfs2_dlm_seq_show(). That is fine for the inline fields, but the
userspace fsdlm stack stores the LVB through lksb_fsdlm.sb_lvbptr. Once
the iterator drops the tracking lock, a copied non-NULL sb_lvbptr still
points into the original lockres owner, so teardown can free that
container before the debugfs dump walks the raw LVB bytes.

Rebase the copied sb_lvbptr to the copied l_lksb before dumping the raw
LVB. The seq snapshot already carries the inline LVB storage reserved in
struct ocfs2_dlm_lksb, so the debugfs reader can dump the copied bytes
without borrowing the original lockres lifetime.

The buggy scenario involves two paths, with each column showing the order
within that path:

locking_state reader: lockres teardown:
1. ocfs2_dlm_seq_start()/next() 1. file release or another owner
copies struct ocfs2_lock_res teardown reaches
2. ocfs2_dlm_seq_show() formats ocfs2_lock_res_free()
the copied row 2. the lockres is removed from the
3. ocfs2_dlm_lvb() follows the tracking list
copied sb_lvbptr 3. the owner frees the original
lockres container

Validation reproduced this kernel report:
KASAN slab-use-after-free in ocfs2_dlm_seq_show+0x1bd/0x430
RIP: 0033:0x7f8ec4b1e29d
The buggy address belongs to the object at ffff88810a1e0800 which belongs
to the cache kmalloc-1k of size 1024
The buggy address is located 368 bytes inside of freed 1024-byte region
[ffff88810a1e0800, ffff88810a1e0c00)
Read of size 1
Call trace:
dump_stack_lvl+0x66/0xa0
print_report+0xce/0x630
ocfs2_dlm_seq_show+0x1bd/0x430 (fs/ocfs2/dlmglue.c:3137)
srso_alias_return_thunk+0x5/0xfbef5
__virt_addr_valid+0x19f/0x330
kasan_report+0xe0/0x110
seq_read_iter+0x29d/0x790
seq_read+0x20a/0x280
find_held_lock+0x2b/0x80
rcu_read_unlock+0x18/0x70
full_proxy_read+0x9e/0xd0
vfs_read+0x12c/0x590
ksys_read+0xd2/0x170
do_user_addr_fault+0x65a/0x890
do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Allocated by task stack:
kasan_save_stack+0x33/0x60
kasan_save_track+0x14/0x30
__kasan_kmalloc+0xaa/0xb0
ocfs2_file_open+0x13e/0x300
do_dentry_open+0x233/0x7f0
vfs_open+0x5a/0x1b0
path_openat+0x66d/0x1540
do_file_open+0x186/0x2b0
do_sys_openat2+0xce/0x150
__x64_sys_openat+0xd0/0x140
do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task stack:
kasan_save_stack+0x33/0x60
kasan_save_track+0x14/0x30
kasan_save_free_info+0x3b/0x60
__kasan_slab_free+0x5f/0x80
kfree+0x313/0x590
ocfs2_file_release+0x138/0x260
__fput+0x1df/0x4b0
fput_close_sync+0xd2/0x170
__x64_sys_close+0x55/0x90
do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
entry_SYSCALL_64_after_hwframe+0x77/0x7f