UBIFS: fix budgeting request alignment in xattr code

Data length has to be aligned in the budgeting request. Code
in xattr.c did not do this.

Signed-off-by: Zoltan Sogor <weth@inf.u-szeged.hu>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>