UBIFS: TNC / GC race fixes

- update GC sequence number if any nodes may have been moved
even if GC did not finish the LEB
- don't ignore error return when reading

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>