perf_event_open.2: srcfix: add git commit references

perf_event_open.2: srcfix: add git commit references

I noticed you were adding git commit references to the various
Linux version markers.

This adds git commit references for all Linux kernel version
notes in perf_event_open.2

mtk: I backed out two pieces of Vince's patch that were not
source comments. They can be dealt with as separate commits.

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>