perf evlist: Do not use hard coded value for a mmap_pages default

So far what is in there by default is what we were using: 512KB + the
control page, but the admin may change that, and if it does to a smaller
value, all calls to tooling for non root users start failing, requiring
that the user manually set --mmap_pages/-m.

Use instead what is in /proc/sys/kernel/perf_event_mlock_kb.

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-2f6mtm8xu3wo5lhkql6jdblh@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>