Summary:
BPF_STACK_TRACE(_name, _size) will allocate space for _size stack traces
(see https://github.com/torvalds/linux/blob/master/kernel/bpf/stackmap.c#L30-L50).

If we've already used all of this space, subsequent calls to bpf_get_stackid()
will return -ENOMEM (see https://github.com/torvalds/linux/blob/master/kernel/bpf/stackmap.c#L173-L176).

This causes our BPF bytecode to store this value in key_t.stack_id and
subsequently causes our python application to crash due to a KeyError when
invoking stack_traces.walk(k.stack_id).

Let's avoid calling stack_traces.walk(k.stack_id) with back stackid's

Test Plan:
Run offcputime.py in an extreme case; with space for only a single stack trace
```
devbig680[bcc](tools): sed_in_file 's/BPF_STACK_TRACE(stack_traces, 1024)/BPF_STACK_TRACE(stack_traces, 1)/' tools/offcputime.py && \
> ~/bcc_run_tool.sh offcputime -f 5; \
> git reset --hard HEAD
swapper/30;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 496
swapper/26;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 553
swapper/28;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 604
swapper/31;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 692
swapper/23;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 713
swapper/18;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 919
swapper/16;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1051
swapper/20;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1056
swapper/21;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1585
swapper/24;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1597
swapper/27;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1610
swapper/17;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1674
swapper/22;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2390
swapper/25;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2574
swapper/19;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2589
swapper/29;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 8428
swapper/8;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15272
swapper/15;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15591
swapper/11;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 17934
swapper/9;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 18100
swapper/14;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 18266
swapper/10;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20124
swapper/12;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20887
swapper/13;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 23453
swapper/3;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 27296
swapper/5;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 29094
swapper/6;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 29799
swapper/7;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 31522
swapper/1;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 32269
swapper/4;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 35585
swapper/2;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 37627
WARNING: 249 stack traces could not be displayed. Consider increasing stack trace storage size.
HEAD is now at d3365e9 [RFC] handle ENOMEM in tools/offcputime`
```