LU-17950 ldiskfs: race in ext4_inode_attach_jinode

LU-17950 ldiskfs: race in ext4_inode_attach_jinode

A race condition could happen when multiple threads
trying to attach jinode for the same inode:

Thread 1:
ext4_map_blocks
  ext4_inode_attach_jinode
    spin_lock(&inode->i_lock)
    ei->jinode = jinode
->
    jbd2_journal_init_jbd_inode(ei->jinode, inode)

Thread 2:
ext4_map_blocks
  ext4_inode_attach_jinode
    if (ei->jinode || !EXT4_SB(inode->i_sb)->s_journal)
	    return 0;
  ext4_jbd2_inode_add_write
->  jbd2_journal_file_inode

The problem is in ext4_inode_attach_jinode() the initial check
of ei->jinode is not protected by inode->i_lock,
thread 2 could go ahead and use the not yet initialized jinode
in jbd2_journal_file_inode(), and thread 1 later will
use jbd2_journal_init_jbd_inode, corrupting the jinode.

Note this issue is specific to ldiskfs because of
ext4-attach-jinode-in-writepages.patch added
ext4_inode_attach_jinode() to make sure jinode is initialized
before calling ext4_jbd2_inode_add_write().

Change-Id: Iafd7aa9537505afbf4bc53fef40ea3aa0a94b7da
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58381
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>