UBIFS: do not look up truncation nodes

When moving nodes in GC, do not try to look up truncation nodes in TNC,
because they do not exist there. This would be harmless, because the TNC
look-up would fail, if we did not have bug 'ubifs_add_snod()' which reads
garbage into 'snod->key'. But in any case, it is less error prone to
explicitly ignore everything but inode, data, dentry and xentry nodes.

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