biolatency: Fix --disks bpf_probe_read() (#1980)

bpf_probe_read()'s third argument is no longer rewritten. Instead, use a
temporary variable (like #1973) to avoid a memory access error.

Before:

```
$ sudo biolatency -D 1
bpf: Failed to load program: Permission denied
0: (79) r1 = *(u64 *)(r1 +112)
1: (7b) *(u64 *)(r10 -8) = r1
[..]
R1 invalid mem access 'inv'

HINT: The invalid mem access 'inv' error can happen if you try to
dereference memory without first using bpf_probe_read() to copy it to
the BPF stack. Sometimes the bpf_probe_read is automatic by the bcc
rewriter, other times you'll need to be explicit.
```

After, works as expected.