UBIFS: re-calculate min_idx_size after the commit

When we commit, but before we try to write anything to the flash
media, @c->min_idx_size is inaccurate, because we do not re-calculate
it after the commit. Do not forget to do this.

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