ext4: cleanup ext4_sync_parent()

A condition !hlist_empty(&inode->i_dentry) is always true for open file.
Just remove it. Also ext4_sync_parent() could use some explanation why
races with rmdir() are not an issue - add a comment explaining that.
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/fsync.c

@@ -61,6 +61,13 @@ static int ext4_sync_parent(struct inode *inode)
 			break;
 		iput(inode);
 		inode = next;
+		/*
+		 * The directory inode may have gone through rmdir by now. But
+		 * the inode itself and its blocks are still allocated (we hold
+		 * a reference to the inode so it didn't go through
+		 * ext4_evict_inode()) and so we are safe to flush metadata
+		 * blocks and the inode.
+		 */
 		ret = sync_mapping_buffers(inode->i_mapping);
 		if (ret)
 			break;
@@ -107,7 +114,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 
 	if (!journal) {
 		ret = __generic_file_fsync(file, start, end, datasync);
-		if (!ret && !hlist_empty(&inode->i_dentry))
+		if (!ret)
 			ret = ext4_sync_parent(inode);
 		if (test_opt(inode->i_sb, BARRIER))
 			goto issue_flush;