perf: bug in rdpmc/mmap accounting after exec

[Vince Weaver <vincent.weaver@maine.edu>]

Hello

so one last bug found by the PAPI testsuite.

This one involves the rdpmc auto-disable on last unmap of an event 
feature.

Failing test case:

	fd=perf_event_open();
	addr=mmap(fd);
	exec()  // without closing or unmapping the event
	fd=perf_event_open();
	addr=mmap(fd);
	rdpmc()	// GPFs due to rdpmc being disabled

I won't pretend to be able to follow the rdpmc disabling code, but if I 
add some printks it looks like 
	current->mm->context.perf_rdpmc_allowed
isn't properly being reset on exec?

In fact, current->mm->context.perf_rdpmc_allowed goes negative which seems
like it shouldn't happen?

Anyway, a test case for this can be found in the perf_event_tests,
	tests/rdpmc/rdpmc_exec_papi

Vince