libperf: Add perf_evlist_mmap_ops::get callback

libperf: Add perf_evlist_mmap_ops::get callback

Add the perf_evlist_mmap_ops::get callback to be called in
mmap_per_evsel() to get/allocate the 'struct perf_mmap' object.

Add the libperf's perf_evlist__mmap_cb_get() function as libperf's get
callback.

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/20191007125344.14268-17-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>