UBIFS: fix a memory leak on error path.

In 'mount_ubifs()', in case of 'ubifs_leb_unmap()' falure,
free allocated resources.

Signed-off-by: Matthieu CASTET <matthieu.castet@parrot.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>