It becomes a perf_mmap method, "push", that build reads from a mmap and
"pushes" it to a consumer, that in the initial case, for 'perf record',
just writes it to the perf.data file descriptor, but may be used by
'top', etc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-u4l1qjbi6l76r2k0nv99220n@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>