selftests/bpf: Fix set but not used build errors

There are some set but not used build errors when compiling bpf selftests
with the latest upstream mainline GCC, at the beginning add the attribute
__maybe_unused for the variables, but it is better to just add the option
-Wno-unused-but-set-variable to CFLAGS in Makefile to disable the errors
instead of hacking the tests.

  tools/testing/selftests/bpf/map_tests/lpm_trie_map_basic_ops.c:229:36:
  error: variable ‘n_matches_after_delete’ set but not used [-Werror=unused-but-set-variable=]

  tools/testing/selftests/bpf/map_tests/lpm_trie_map_basic_ops.c:229:25:
  error: variable ‘n_matches’ set but not used [-Werror=unused-but-set-variable=]

  tools/testing/selftests/bpf/prog_tests/bpf_cookie.c:426:22:
  error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

  tools/testing/selftests/bpf/prog_tests/find_vma.c:52:22:
  error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

  tools/testing/selftests/bpf/prog_tests/perf_branches.c:67:22:
  error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

  tools/testing/selftests/bpf/prog_tests/perf_link.c:15:22:
  error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

Cc: [email protected]
Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Tiezhu Yang <[email protected]>

Author: seehearfeel

tools/testing/selftests/bpf/Makefile

@@ -46,6 +46,7 @@ endif
 
 CFLAGS += -g $(OPT_FLAGS) -rdynamic -std=gnu11				\
 	  -Wall -Werror -fno-omit-frame-pointer				\
+	  -Wno-unused-but-set-variable					\
 	  $(GENFLAGS) $(SAN_CFLAGS) $(LIBELF_CFLAGS)			\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
 	  -I$(TOOLSINCDIR) -I$(TOOLSARCHINCDIR) -I$(APIDIR) -I$(OUTPUT)