bpf_probe_read() is often used to access pointees in bpf programs.
Recent rewriter has become smarter so a lot of bpf_probe_read()
can be replaced with simple pointer/member access.

In certain cases, bpf_probe_read() is still preferred though.
For example, kernel net/tcp.h defined TCP_SKB_CB as below
  #define TCP_SKB_CB(__skb)	((struct tcp_skb_cb *)&((__skb)->cb[0]))
User can use below to access tcp_gso_size of a skb data structure.
  TCP_SKB_CB(skb)->tcp_gso_size
The rewriter will fail as it attempts to rewrite (__skb)->cb[0].

Instead of chasing down to prevent exactly the above pattern,
this patch detects function bpf_probe_read() in ProbeVisitor and
will skip it so bpf_probe_read()'s third parameter is a AddrOf.
This can also help other cases where rewriter is not
capable and user used bpf_probe_read() as the workaround.

Also fixed tcptop.py to use direct assignment instead of
bpf_probe_read. Otherwise, rewriter will actually rewrite
src address reference inside the bpf_probe_read().
Signed-off-by: Yonghong Song <yhs@fb.com>