selftests/bpf: Fix a few compiler warnings

With clang building selftests/bpf, I hit a few warnings like below:

  .../bpf_iter.c:592:48: warning: variable 'expected_key_c' set but not used [-Wunused-but-set-variable]
  __u32 expected_key_a = 0, expected_key_b = 0, expected_key_c = 0;
                                                ^

  .../bpf_iter.c:688:48: warning: variable 'expected_key_c' set but not used [-Wunused-but-set-variable]
  __u32 expected_key_a = 0, expected_key_b = 0, expected_key_c = 0;
                                                ^

  .../tc_redirect.c:657:6: warning: variable 'target_fd' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  if (!ASSERT_OK_PTR(nstoken, "setns " NS_FWD))
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  .../tc_redirect.c:743:6: note: uninitialized use occurs here
  if (target_fd >= 0)
      ^~~~~~~~~

Removing unused variables and initializing the previously-uninitialized variable
to ensure these warnings are gone.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210917043343.3711917-1-yhs@fb.com