f2fs: fix to set/clear I_LINKABLE under i_lock

fsstress + fault injection test case reports a warning message as
below:

WARNING: CPU: 13 PID: 6226 at fs/inode.c:361 inc_nlink+0x32/0x40
Call Trace:
 f2fs_init_inode_metadata+0x25c/0x4a0 [f2fs]
 f2fs_add_inline_entry+0x153/0x3b0 [f2fs]
 f2fs_add_dentry+0x75/0x80 [f2fs]
 f2fs_do_add_link+0x108/0x160 [f2fs]
 f2fs_rename2+0x6ab/0x14f0 [f2fs]
 vfs_rename+0x70c/0x940
 do_renameat2+0x4d8/0x4f0
 __x64_sys_renameat2+0x4b/0x60
 do_syscall_64+0x33/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Following race case can cause this:
Thread A				Kworker
- f2fs_rename
 - f2fs_create_whiteout
  - __f2fs_tmpfile
   - f2fs_i_links_write
    - f2fs_mark_inode_dirty_sync
     - mark_inode_dirty_sync
					- writeback_single_inode
					 - __writeback_single_inode
					  - spin_lock(&inode->i_lock)
   - inode->i_state |= I_LINKABLE
					  - inode->i_state &= ~dirty
					  - spin_unlock(&inode->i_lock)
 - f2fs_add_link
  - f2fs_do_add_link
   - f2fs_add_dentry
    - f2fs_add_inline_entry
     - f2fs_init_inode_metadata
      - f2fs_i_links_write
       - inc_nlink
        - WARN_ON(!(inode->i_state & I_LINKABLE))

Fix to add i_lock to avoid i_state update race condition.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/namei.c

@@ -855,7 +855,11 @@ static int __f2fs_tmpfile(struct inode *dir, struct dentry *dentry,
 
 	if (whiteout) {
 		f2fs_i_links_write(inode, false);
+
+		spin_lock(&inode->i_lock);
 		inode->i_state |= I_LINKABLE;
+		spin_unlock(&inode->i_lock);
+
 		*whiteout = inode;
 	} else {
 		d_tmpfile(dentry, inode);
@@ -1041,7 +1045,11 @@ static int f2fs_rename(struct inode *old_dir, struct dentry *old_dentry,
 		err = f2fs_add_link(old_dentry, whiteout);
 		if (err)
 			goto put_out_dir;
+
+		spin_lock(&whiteout->i_lock);
 		whiteout->i_state &= ~I_LINKABLE;
+		spin_unlock(&whiteout->i_lock);
+
 		iput(whiteout);
 	}