blkdebug: Refactor error injection

Rather than repeat the logic at each caller of checking if a Rule
exists that warrants an error injection, fold that logic into
inject_error(); and rename it to rule_check() for legibility.
This will help the next patch, which adds two more callers that
need to check rules for the potential of injecting errors.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170429191419.30051-6-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
