selftests/bpf: Fix clang compilation errors

llvm upstream patch ([1]) added to issue warning for code like
  void test() {
    int j = 0;
    for (int i = 0; i < 1000; i++)
            j++;
    return;
  }

This triggered several errors in selftests/bpf build since
compilation flag -Werror is used.
  ...
  test_lpm_map.c:212:15: error: variable 'n_matches' set but not used [-Werror,-Wunused-but-set-variable]
        size_t i, j, n_matches, n_matches_after_delete, n_nodes, n_lookups;
                     ^
  test_lpm_map.c:212:26: error: variable 'n_matches_after_delete' set but not used [-Werror,-Wunused-but-set-variable]
        size_t i, j, n_matches, n_matches_after_delete, n_nodes, n_lookups;
                                ^
  ...
  prog_tests/get_stack_raw_tp.c:32:15: error: variable 'cnt' set but not used [-Werror,-Wunused-but-set-variable]
        static __u64 cnt;
                     ^
  ...

  For test_lpm_map.c, 'n_matches'/'n_matches_after_delete' are changed to be volatile
  in order to silent the warning. I didn't remove these two declarations since
  they are referenced in a commented code which might be used by people in certain
  cases. For get_stack_raw_tp.c, the variable 'cnt' is removed.

  [1] https://reviews.llvm.org/D122271



Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220325200304.2915588-1-yhs@fb.com