ext4: track writeback errors using the generic tracking infrastructure

We already using mapping_set_error() in fs/ext4/page_io.c, so all we
need to do is to use file_check_and_advance_wb_err() when handling
fsync() requests in ext4_sync_file().
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/fsync.c

@@ -164,6 +164,9 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 			ret = err;
 	}
 out:
+	err = file_check_and_advance_wb_err(file);
+	if (ret == 0)
+		ret = err;
 	trace_ext4_sync_file_exit(inode, ret);
 	return ret;
 }