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: f131d9edc2 ("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>

tools/testing/selftests/lkdtm/run.sh

@@ -82,7 +82,7 @@ dmesg > "$DMESG"
 ($SHELL -c 'cat <(echo '"$test"') >'"$TRIGGER" 2>/dev/null) || true
 
 # Record and dump the results
-dmesg | diff --changed-group-format='%>' --unchanged-group-format='' "$DMESG" - > "$LOG" || true
+dmesg | comm --nocheck-order -13 "$DMESG" - > "$LOG" || true
 
 cat "$LOG"
 # Check for expected output