UBIFS: switch to ubifs_leb_read

Instead of using 'ubi_read()' function directly, used the 'ubifs_leb_read()'
helper function instead. This allows to get rid of several redundant error
messages and make sure that we always have a stack dump on read errors.

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