UBIFS: align inode data to eight

UBIFS aligns node lengths to 8, so budgeting has to do the
same. Well, direntry, inode, and page budgets are already
aligned, but not inode data budget (e.g., data in special
devices or symlinks). Do this for inode data as well.
Also, add corresponding debugging checks.

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