UBIFS: fix false assertion warning in case of I/O failures

When UBIFS switches to R/O mode because it detects I/O failures, then
when we unmount, we still may have allocated budget, and the assertions
which verify that we have not budget will fire. But it is expected to
have the budget in case of I/O failures, so the assertion warnings will
be false. Suppress them for the I/O failure case.

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