UBIFS: restore budg_uncommitted_idx

UBIFS stores uncommitted index size in c->budg_uncommitted_idx,
and this affect budgeting calculations. When mounting and
replaying, this variable is not updated, so we may end up
with "over-budgeting". This patch fixes the issue.

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