perf_counter: sysctl for system wide perf counters

Impact: add sysctl for paranoid/relaxed perfcounters policy

Allow the use of system wide perf counters to everybody, but provide
a sysctl to disable it for the paranoid security minded.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090409085524.514046352@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>