tools lib: Add bitmap_alloc function

Adding bitmap_alloc function to dynamically allocate bitmap.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20160802113302.GA7479@krava


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>