Fix filter intended to exclude perf's own writes.

[kelleyk]

I noticed that the write_workaround code paths weren't working as intended (which is bad, since perf recording its own writes causes something of a feedback loop).

The idea behind the trick being used is that sh substitutes its PID for $$; then exec replaces sh with perf (keeping the same PID), so we have managed to put perf's own PID in the filter, which successfully excludes perf's writes. (Yay!)

This patch just restores the execs, which must have gone missing at some point, and without which we are excluding sh's writes instead.