fix tcplife.py rewriter issue

rewriter tried to rewrite an argument for a user written
bpf_probe_read and triggers a clang compilation error.

  $ tcplife.py
  /virtual/main.c:134:41: error: cannot take the address of an rvalue of type 'typeof(u64)' (aka 'unsigned long long')
    ...&({ typeof(u64) _val; __builtin_memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val), (u64)&tp->bytes_received); _val; }));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /virtual/main.c:135:41: error: cannot take the address of an rvalue of type 'typeof(u64)' (aka 'unsigned long long')
    ...&({ typeof(u64) _val; __builtin_memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val), (u64)&tp->bytes_acked); _val; }));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  2 errors generated.

changing bpf_probe_read to regular pointer access fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>