ext4, dax: set ext4_dax_aops for dax files

Sync syscall to DAX file needs to flush processor cache, but it
currently does not flush to existing DAX files.  This is because
'ext4_da_aops' is set to address_space_operations of existing DAX
files, instead of 'ext4_dax_aops', since S_DAX flag is set after
ext4_set_aops() in the open path.

  New file
  --------
  lookup_open
    ext4_create
      __ext4_new_inode
        ext4_set_inode_flags   // Set S_DAX flag
      ext4_set_aops            // Set aops to ext4_dax_aops

  Existing file
  -------------
  lookup_open
    ext4_lookup
      ext4_iget
        ext4_set_aops          // Set aops to ext4_da_aops
        ext4_set_inode_flags   // Set S_DAX flag

Change ext4_iget() to initialize i_flags before ext4_set_aops().

Fixes: 5f0663bb4a64 ("ext4, dax: introduce ext4_dax_aops")
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Suggested-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org

fs/ext4/inode.c

@@ -4896,6 +4896,7 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 		 * not initialized on a new filesystem. */
 	}
 	ei->i_flags = le32_to_cpu(raw_inode->i_flags);
+	ext4_set_inode_flags(inode);
 	inode->i_blocks = ext4_inode_blocks(raw_inode, ei);
 	ei->i_file_acl = le32_to_cpu(raw_inode->i_file_acl_lo);
 	if (ext4_has_feature_64bit(sb))
@@ -5042,7 +5043,6 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 		goto bad_inode;
 	}
 	brelse(iloc.bh);
-	ext4_set_inode_flags(inode);
 
 	unlock_new_inode(inode);
 	return inode;