selftests/powerpc/pmu: Use signed long to read perf_event_paranoid

Excerpt from man 2 perf_event_open:

  /proc/sys/kernel/perf_event_paranoid
  The perf_event_paranoid file can be set to restrict access to the
  performance counters.
    2 allow only user-space measurements.
    1 allow both kernel and user measurements (default).
    0 allow access to CPU-specific data but not raw tracepoint samples.
   -1 no restrictions.

require_paranoia_below() should return 0 if perf_event_paranoid is below
a specified level, the value from perf_event_paranoid is read into an
unsigned long so the incorrect value is returned when
perf_event_paranoid is set to -1.

Without this patch applied there is the same number of selftests/powerpc
which skip when /proc/sys/kernel/perf_event_paranoid is set to 1 or -1
but no skips when set to zero.

With this patch applied there are no skipped selftests/powerpc test when
/proc/sys/kernel/perf_event_paranoid is set to 0 or -1.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>