This commit adds support for multiple consecutive and nested pointer
dereference of function arguments that should be converted to
bpf_probe_read. The logic works by marking variables as needing a
probe_read if they come from the register argument, and then applying
this property transitively.

Supported syntax:
```
int trace_entry(struct pt_regs *ctx, struct file *file) {
    struct vfsmount *mnt = file->f_path.mnt;
    struct super_block *k = mnt->mnt_sb;
    const char *name = file->f_path.dentry->d_name.name;
```

Not supported: probe reads from map leaves, probe reads after explicit casts.

Fixes: #188
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>