block: Fix multiwrite memory leak in error case Previously multiwrite_user_cb was never called if a request in the multiwrite batch failed right away because it did set mcb->error immediately. Make it look more like a normal callback to fix this. Reported-by: Juan Quintela <quintela@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>