perf trace: Let the perf_evlist__mmap autosize the number of pages to use

perf trace: Let the perf_evlist__mmap autosize the number of pages to use

So that normal users can run 'trace', we were using a hardcoded 1024
pages value that was more than the default /proc/sys/kernel/perf_event_mlock_kb
setting.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-3banj3yh0sjz41obxtgiel3a@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>