libperf: Adopt perf_mmap__read_done() from tools/perf

Move perf_mmap__read_init() from tools/perf to libperf and export it in
the perf/mmap.h header.

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/20191007125344.14268-12-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/arch/x86/tests/perf-time-to-tsc.c

@@ -142,7 +142,7 @@ int test__perf_time_to_tsc(struct test *test __maybe_unused, int subtest __maybe
 next_event:
 			perf_mmap__consume(&md->core);
 		}
-		perf_mmap__read_done(md);
+		perf_mmap__read_done(&md->core);
 	}
 
 	if (!comm1_time || !comm2_time)