perf mmap: Adopt push method from builtin-record.c

perf mmap: Adopt push method from builtin-record.c

The previous prep patch was just to show exactly what changed in that
function, now its time to move that method and things only it uses to
the right place, mmap.[ch]

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-aaxywfgw3d44x6xlu8zm1avu@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>