ubifs: Fix memory leak on error condition

If the call to ubifs_read_nnode() fails in ubifs_lpt_calc_hash() an
error is returned without freeing the memory allocated to 'buf'.
Read and check the root node before allocating the buffer.

Detected by CoverityScan, CID 1441025 ("Resource leak")

Signed-off-by: Garry McNulty <garrmcnu@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>