[CI][linux-kernel] Enable -Wunused-const-variable and -Wunused-but-set-variable

.github/workflows/linux-kernel.yml

@@ -10,4 +10,4 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Create linux kernel library + build + test
-      run: make -C contrib/linux-kernel test
+      run: make -C contrib/linux-kernel test CFLAGS="-Werror -Wunused-const-variable -Wunused-but-set-variable"

contrib/linux-kernel/Makefile

@@ -65,7 +65,7 @@ import: libzstd
 
 .PHONY: test
 test: libzstd
-	$(MAKE) -C test run-test CFLAGS="-O3" -j
+	$(MAKE) -C test run-test CFLAGS="-O3 $(CFLAGS)" -j
 
 .PHONY: clean
 clean: