libperf: Centralize map refcnt setting

Currently when a new map is mmapped we set its refcnt to 2 in the
perf_evlist_mmap_ops::mmap callback.

Every mmap gets its refcnt set to 2 when it's first mmaped:

  - 1 for the current user, which will be taken out by a call to
    perf_evlist__munmap_filtered(), where we find out there's
    no more data comming from kernel to this mmap.

  - 1 for the drain code where in perf_mmap__consume() the mmap
    is released if it is empty.

Move this common setup into libperf's generic code before the mmap
callback is called.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20191007125344.14268-23-jolsa@kernel.org


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