UBIFS: switch to RO mode after synchronizing

In 'ubifs_garbage_collect()' on error path, we first switch to R/O mode, and
then synchronize write-buffers (to make sure no data are lost). But the GC
write-buffer synchronization will fail, because we are already in R/O mode.
This patch re-orders this and makes sure we first synchronize the write-buffer,
and then switch to R/O mode.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/gc.c

@@ -774,8 +774,8 @@ out_unlock:
 out:
 	ubifs_assert(ret < 0);
 	ubifs_assert(ret != -ENOSPC && ret != -EAGAIN);
-	ubifs_ro_mode(c, ret);
 	ubifs_wbuf_sync_nolock(wbuf);
+	ubifs_ro_mode(c, ret);
 	mutex_unlock(&wbuf->io_mutex);
 	ubifs_return_leb(c, lp.lnum);
 	return ret;