ubifs: Set/Clear I_LINKABLE under i_lock for whiteout inode

xfstests-generic/476 reports a warning message as below:

WARNING: CPU: 2 PID: 30347 at fs/inode.c:361 inc_nlink+0x52/0x70
Call Trace:
  do_rename+0x502/0xd40 [ubifs]
  ubifs_rename+0x8b/0x180 [ubifs]
  vfs_rename+0x476/0x1080
  do_renameat2+0x67c/0x7b0
  __x64_sys_renameat2+0x6e/0x90
  do_syscall_64+0x66/0xe0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Following race case can cause this:
         rename_whiteout(Thread 1)             wb_workfn(Thread 2)
ubifs_rename
  do_rename
                                          __writeback_single_inode
					    spin_lock(&inode->i_lock)
    whiteout->i_state |= I_LINKABLE
                                            inode->i_state &= ~dirty;
---- How race happens on i_state:
    (tmp = whiteout->i_state | I_LINKABLE)
		                           (tmp = inode->i_state & ~dirty)
    (whiteout->i_state = tmp)
		                           (inode->i_state = tmp)
----
					    spin_unlock(&inode->i_lock)
    inc_nlink(whiteout)
    WARN_ON(!(inode->i_state & I_LINKABLE)) !!!

Fix to add i_lock to avoid i_state update race condition.

Fixes: 9e0a1fff ("ubifs: Implement RENAME_WHITEOUT")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/dir.c

@@ -1337,7 +1337,10 @@ static int do_rename(struct inode *old_dir, struct dentry *old_dentry,
 			goto out_release;
 		}
 
+		spin_lock(&whiteout->i_lock);
 		whiteout->i_state |= I_LINKABLE;
+		spin_unlock(&whiteout->i_lock);
+
 		whiteout_ui = ubifs_inode(whiteout);
 		whiteout_ui->data = dev;
 		whiteout_ui->data_len = ubifs_encode_dev(dev, MKDEV(0, 0));
@@ -1430,7 +1433,11 @@ static int do_rename(struct inode *old_dir, struct dentry *old_dentry,
 
 		inc_nlink(whiteout);
 		mark_inode_dirty(whiteout);
+
+		spin_lock(&whiteout->i_lock);
 		whiteout->i_state &= ~I_LINKABLE;
+		spin_unlock(&whiteout->i_lock);
+
 		iput(whiteout);
 	}