block: add empty account cookie type

block: add empty account cookie type

Each block_acct_done/failed call is designed to correspond to a
previous block_acct_start call, which initializes the stats cookie.
However sometimes it is not the case, e.g. some error paths might
report the same cookie twice because it is hard to accurately track if
the cookie was reported yet or not.

This patch cleans the cookie after report.
(Note: block_acct_failed/done without a previous block_acct_start at
all should be avoided. Uninitialized cookie might hold a garbage value
and there is still "< BLOCK_MAX_IOTYPE" assertion for that)

It will be particularly useful in ide code where it's hard to
keep track whether the request done its accounting or not: in the
following patch of the series, trim requests will do the accounting
separately.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190923121737.83281-4-anton.nefedov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>