perf trace: Bump --mmap-pages when --call-graph is used by the root user

perf trace: Bump --mmap-pages when --call-graph is used by the root user

To reduce the chances we'll overflow the mmap buffer, manual fine tuning
trumps this.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-wxygbxmp1v9mng1ea28wet02@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>