perf_counter, x86: rename bitmasks to ->used_mask and ->active_mask

Standardize on explicitly mentioning '_mask' in fields that
are not plain flags but masks. This avoids typos like:

       if (cpuc->used)

(which could easily slip through review unnoticed), while if a
typo looks like this:

       if (cpuc->used_mask)

it might get noticed during review.

[ Impact: cleanup ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1241016956-24648-1-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>