UBIFS: do not write rubbish into truncation scanning node

In the scanning code, in 'ubifs_add_snod()', we write rubbish into
'snod->key', because we assume that on-flash truncation nodes have a key, but
they do not. If the other parts of UBIFS then mistakenly try to look-up
the truncation node key (they should not do this, but may do because of a bug),
we can succeed and corrupt TNC. It looks like we did have such a situation in
'sort_nodes()' in gc.c.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/scan.c

@@ -212,7 +212,6 @@ int ubifs_add_snod(const struct ubifs_info *c, struct ubifs_scan_leb *sleb,
 	case UBIFS_DENT_NODE:
 	case UBIFS_XENT_NODE:
 	case UBIFS_DATA_NODE:
-	case UBIFS_TRUN_NODE:
 		/*
 		 * The key is in the same place in all keyed
 		 * nodes.