ext4: check return value of blkdev_issue_flush()

blkdev_issue_flush() can fail; make sure the error gets properly
propagated.
    
This is a port of the equivalent ext3 patch from commit 44f4f729e7.
    
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/fsync.c

@@ -203,7 +203,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	struct inode *inode = file->f_mapping->host;
 	struct ext4_inode_info *ei = EXT4_I(inode);
 	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
-	int ret;
+	int ret, err;
 	tid_t commit_tid;
 	bool needs_barrier = false;
 
@@ -255,8 +255,11 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 		needs_barrier = true;
 	jbd2_log_start_commit(journal, commit_tid);
 	ret = jbd2_log_wait_commit(journal, commit_tid);
-	if (needs_barrier)
-		blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL, NULL);
+	if (needs_barrier) {
+		err = blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL, NULL);
+		if (!ret)
+			ret = err;
+	}
  out:
 	mutex_unlock(&inode->i_mutex);
 	trace_ext4_sync_file_exit(inode, ret);