blkdebug: fix "once" rule

Background:
  The blkdebug scripts are currently engineered so that when a debug
event occurs, a prefilter browses a master list of parsed rules for a
certain event and adds them to an "active list" of rules to be used for
the forthcoming action, provided the events and state numbers match.

  Then, once the request is received, the last active rule is used to
inject an error if certain parameters match.

  This active list is cleared every time the prefilter injects a new
rule for the first time during a debug event.

  The "once" rule currently causes the error injection, if it is
triggered, to only clear the active list. This is insufficient for
preventing future injections of the same rule.

Remedy:
  This patch /deletes/ the rule from the list that the prefilter
browses, so it is gone for good. In V2, we remove only the rule of
interest from the active list instead of allowing the "once" rule to
clear the entire list of active rules.

Impact:
  This affects iotests 026. Several ENOSPC tests that used "once" can
be seen to have output that shows multiple failure messages. After
this patch, the error messages tend to be smaller and less severe, but
the injection can still be seen to be working. I have patched the
expected output to expect the smaller error messages.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1423257977-25630-1-git-send-email-jsnow@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
