selftests/bpf: Silence unused-but-set build warnings

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=]

Signed-off-by: Tiezhu Yang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[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)