Add a test case to ensure that released pointer registers will not be
leaked into the map.

Before fix:

  ./test_verifier 984
    984/u reference tracking: try to leak released ptr reg FAIL
    Unexpected success to load!
    verification time 67 usec
    stack depth 4
    processed 23 insns (limit 1000000) max_states_per_insn 0 total_states 2
    peak_states 2 mark_read 1
    984/p reference tracking: try to leak released ptr reg OK
    Summary: 1 PASSED, 0 SKIPPED, 1 FAILED

After fix:

  ./test_verifier 984
    984/u reference tracking: try to leak released ptr reg OK
    984/p reference tracking: try to leak released ptr reg OK
    Summary: 2 PASSED, 0 SKIPPED, 0 FAILED
Signed-off-by: Youlin Li <liulin063@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221103093440.3161-2-liulin063@gmail.com