libperf: Introduce perf_evlist__mmap_ops()

To be able to pass specific callbacks to evlist's mmap.

There will be a specific call to this function from perf's
evlist__mmap() and libperf's perf_evlist__mmap() functions in following
changes.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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/20191007125344.14268-15-jolsa@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>