UBIFS: introduce a separate structure for budgeting info

This patch separates out all the budgeting-related information
from 'struct ubifs_info' to 'struct ubifs_budg_info'. This way the
code looks a bit cleaner. However, the main driver for this is
that we want to save budgeting information and print it later,
so a separate data structure for this is helpful.

This patch is a preparation for the further debugging output
improvements.

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