trace: Specifying a pid with a kernel probe now works (#589)
trace: Specifying a pid with a kernel probe now works (#589)

Due to an incorrectly referenced global variable, specifying a pid
to filter with a kernel probe produced an error. This is now fixed,
for example:

```
TIME     PID    COMM         FUNC
23:46:00 29967  bash         sched_switch
23:46:01 29967  bash         sched_switch
23:46:01 29967  bash         sched_switch
^C
```
mcaleavya/bcc,tuxology/bcc,mkacik/bcc,brendangregg/bcc,shodoco/bcc,mcaleavya/bcc,romain-intel/bcc,mcaleavya/bcc,tuxology/bcc,brendangregg/bcc,mkacik/bcc,tuxology/bcc,shodoco/bcc,romain-intel/bcc,mkacik/bcc,tuxology/bcc,mkacik/bcc,tuxology/bcc,shodoco/bcc,iovisor/bcc,mkacik/bcc,romain-intel/bcc,mcaleavya/bcc,iovisor/bcc,iovisor/bcc,shodoco/bcc,shodoco/bcc,iovisor/bcc,brendangregg/bcc,romain-intel/bcc,brendangregg/bcc,brendangregg/bcc,romain-intel/bcc,mcaleavya/bcc,iovisor/bcc