selftests/lkdtm: Use "comm" instead of "diff" for dmesg

selftests/lkdtm: Use "comm" instead of "diff" for dmesg

Instead of full GNU diff (which smaller boot environments may not have),
use "comm" which is more available.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: linux-kselftest@vger.kernel.org
Fixes: 7ae48e01456f ("selftests/lkdtm: Don't clear dmesg when running tests")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200909211700.2399399-1-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>