tools/nfsslower.py: Fix uninitialized struct pad error

[Rtoax]

The verifier is unhappy, if data struct pad is not initialized, see [0][1].

    $ sudo ./nfsslower.py
    ...
    ; bpf_perf_event_output(ctx, (void *)bpf_pseudo_fd(1, -2), CUR_CPU_IDENTIFIER, &data, sizeof(data));
    83: (79) r1 = *(u64 *)(r10 -144)      ; R1_w=ctx(off=0,imm=0) R10=fp0
    84: (18) r3 = 0xffffffff              ; R3_w=4294967295
    86: (b7) r5 = 96                      ; R5_w=96
    87: (85) call bpf_perf_event_output#25
    invalid indirect read from stack R4 off -136+92 size 96
    processed 84 insns (limit 1000000) max_states_per_insn 0 total_states 4 peak_states 4 mark_read 4
    ...
    raise Exception("Failed to load BPF program %s: %s" %
    Exception: Failed to load BPF program b'raw_tracepoint__nfs_commit_done': Permission denied

[0] #2623
[1] #4453