ocfs2: fix oops when writing cloned file

ocfs2: fix oops when writing cloned file

Writing a cloned file triggers a kernel oops and the user-space command
process is also killed by the system.  The bug can be reproduced stably
via:

1) create a file under ocfs2 file system directory.

  journalctl -b > aa.txt

2) create a cloned file for this file.

  reflink aa.txt bb.txt

3) write the cloned file with dd command.

  dd if=/dev/zero of=bb.txt bs=512 count=1 conv=notrunc

The dd command is killed by the kernel, then you can see the oops message
via dmesg command.

[  463.875404] BUG: kernel NULL pointer dereference, address: 0000000000000028
[  463.875413] #PF: supervisor read access in kernel mode
[  463.875416] #PF: error_code(0x0000) - not-present page
[  463.875418] PGD 0 P4D 0
[  463.875425] Oops: 0000 [#1] SMP PTI
[  463.875431] CPU: 1 PID: 2291 Comm: dd Tainted: G           OE     5.3.16-2-default
[  463.875433] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[  463.875500] RIP: 0010:ocfs2_refcount_cow+0xa4/0x5d0 [ocfs2]
[  463.875505] Code: 06 89 6c 24 38 89 eb f6 44 24 3c 02 74 be 49 8b 47 28
[  463.875508] RSP: 0018:ffffa2cb409dfce8 EFLAGS: 00010202
[  463.875512] RAX: ffff8b1ebdca8000 RBX: 0000000000000001 RCX: ffff8b1eb73a9df0
[  463.875515] RDX: 0000000000056a01 RSI: 0000000000000000 RDI: 0000000000000000
[  463.875517] RBP: 0000000000000001 R08: ffff8b1eb73a9de0 R09: 0000000000000000
[  463.875520] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[  463.875522] R13: ffff8b1eb922f048 R14: 0000000000000000 R15: ffff8b1eb922f048
[  463.875526] FS:  00007f8f44d15540(0000) GS:ffff8b1ebeb00000(0000) knlGS:0000000000000000
[  463.875529] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  463.875532] CR2: 0000000000000028 CR3: 000000003c17a000 CR4: 00000000000006e0
[  463.875546] Call Trace:
[  463.875596]  ? ocfs2_inode_lock_full_nested+0x18b/0x960 [ocfs2]
[  463.875648]  ocfs2_file_write_iter+0xaf8/0xc70 [ocfs2]
[  463.875672]  new_sync_write+0x12d/0x1d0
[  463.875688]  vfs_write+0xad/0x1a0
[  463.875697]  ksys_write+0xa1/0xe0
[  463.875710]  do_syscall_64+0x60/0x1f0
[  463.875743]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  463.875758] RIP: 0033:0x7f8f4482ed44
[  463.875762] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 80 00 00 00
[  463.875765] RSP: 002b:00007fff300a79d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  463.875769] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f8f4482ed44
[  463.875771] RDX: 0000000000000200 RSI: 000055f771b5c000 RDI: 0000000000000001
[  463.875774] RBP: 0000000000000200 R08: 00007f8f44af9c78 R09: 0000000000000003
[  463.875776] R10: 000000000000089f R11: 0000000000000246 R12: 000055f771b5c000
[  463.875779] R13: 0000000000000200 R14: 0000000000000000 R15: 000055f771b5c000

This regression problem was introduced by commit e74540b28556 ("ocfs2:
protect extent tree in ocfs2_prepare_inode_for_write()").

Link: http://lkml.kernel.org/r/20200121050153.13290-1-ghe@suse.com
Fixes: e74540b28556 ("ocfs2: protect extent tree in ocfs2_prepare_inode_for_write()").
Signed-off-by: Gang He <ghe@suse.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>