prevent bpf_probe_read MemberExpr rewrite if not rewritable (#1827)

For the test case in this patch below,
 #define _(P) ({typeof(P) val = 0; bpf_probe_read(&val, sizeof(val), &P); val;})
 int count_tcp(struct pt_regs *ctx, struct sk_buff *skb) {
     return _(TCP_SKB_CB(skb)->tcp_gso_size);
 }

The clang AST will consider the whole `_(TCP_SKB_CB(skb)->tcp_gso_size)`
as a MemberExpr during AST traversal. However, it will consider
the start location of the member expression not rewritable.
Without this patch, we will get an error like below:
    /virtual/main.c:15:44: error: expected ';' after return statement
    return _(TCP_SKB_CB(skb)->tcp_gso_size)); _val; });
Basically, the start of bpf_probe_read() rewritingg failed but
later part succeeded, so the code becomes uncompilable.

Previously, we did not see such issues, but as rewriter got
more smarter this bug is exposed.

This patch fixed the issue by preventing rewriting the whole
expression if the start location for the member expression is
not rewritable.

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