test/ocf: kill -9 bdevperf in stats.sh

We have encountered several latent failures in test/ocf/stats.sh because
we fail to kill bdevperf with sigterm. We really need to figure out why
we can't always kill bdevperf, but in the meantime we can get this test
passing consistently by sending sigkill to bdevperf.

Change-Id: I6f233f4da79293283e3afaf01fd4baf5e60048bd
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/475315


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>