libperf: Adopt perf_mmap__mmap_len() function from tools/perf

libperf: Adopt perf_mmap__mmap_len() function from tools/perf

Move perf_mmap__mmap_len() from tools/perf wto libperf, it will be used
in the following patches. And rename the existing perf's function to
mmap__mmap_len().

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/20191007125344.14268-4-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>