UBIFS: fix a couple bugs in UBIFS xattr length calculation

UBIFS: fix a couple bugs in UBIFS xattr length calculation

The journal update function did not work for extended attributes properly,
because extended attribute inodes carry the xattr data, and the size of this
data was not taken into account.

Artem: improved commit message, amended the patch a bit.

Signed-off-by: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Acked-by: Brad Mouring <brad.mouring@ni.com>
Acked-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>