perf_counter: provide an mlock threshold

Provide a threshold to relax the mlock accounting, increasing usability.

Each counter gets perf_counter_mlock_kb for free.

[ Impact: allow more mmap buffering ]

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: <20090505155437.112113632@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>