UBIFS: always read hashed-key nodes under TNC mutex

Leaf-nodes that have a hashed key are stored in the
leaf-node-cache (LNC) which is protected by the TNC
mutex.  Consequently, when reading a leaf node with
a hashed key (i.e. directory entries, xattr entries)
the TNC mutex is always required.

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