libperf: Adopt perf_mmap__consume() function from tools/perf

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

Move also the needed helpers perf_mmap__write_tail(),
perf_mmap__read_head() and perf_mmap__empty().

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/20191007125344.14268-10-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/lib/libperf.map

@@ -40,6 +40,7 @@ LIBPERF_0.0.1 {
 		perf_evlist__next;
 		perf_evlist__set_maps;
 		perf_evlist__poll;
+		perf_mmap__consume;
 	local:
 		*;
 };