perf evlist: Introduce perf_evlist__mmap_cb_get()

perf evlist: Introduce perf_evlist__mmap_cb_get()

Add the perf_evlist__mmap_cb_get() function to return 'struct perf_mmap'
object during perf_evlist__mmap_ops() call.

The array of 'struct mmap' is allocated via evlist__alloc_mmap(), in
this callback we simply returns pointer to the base object.

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/20191007125344.14268-20-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>