ubifs: Enable journal replay during mount

Enable ubifs_replay_journal during mount_ubifs, which was
disabled before.

This commit fix an issue with unrecoverable ubifs volumes
after power cut.

Therefor the gc.c is imported now from 1860e37 Linux 3.15

hs: added SPDX-License-Identifier for fs/ubifs/gc.c

Signed-off-by: Anton Habegger <anton.habegger@gmail.com>