fix a rewriter bug for array subscript

additional fix for issue #1850

for the below case in test_clang.py;
  int test(struct pt_regs *ctx, struct mm_struct *mm) {
      return mm->rss_stat.count[MM_ANONPAGES].counter;
  }

the current rewriter generates:
  int test(struct pt_regs *ctx) {
   struct mm_struct *mm = ctx->di;
      return ({ typeof(atomic_long_t) _val;
                __builtin_memset(&_val, 0, sizeof(_val));
                bpf_probe_read(&_val,
                               sizeof(_val),
                               (u64)(&mm->rss_stat.count) + (MM_ANONPAGES));
                _val; }).counter;
  }
The third argument of bpf_probe_read() is incorrect.
The correct third argument should be
   (u64)((&mm->rss_stat.count) + (MM_ANONPAGES))

This patch fixed the issue by adding extra parenthesis for the
outer u64 type casting.

  int test(struct pt_regs *ctx) {
   struct mm_struct *mm = ctx->di;
      return ({ typeof(atomic_long_t) _val;
                __builtin_memset(&_val, 0, sizeof(_val));
                bpf_probe_read(&_val,
                               sizeof(_val),
                               (u64)((&mm->rss_stat.count) + (MM_ANONPAGES)));
                _val; }).counter;
  }

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