UBIFS: do not write rubbish into truncation scanning node

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>