perf tools: Change perf_mem__lvl_scnprintf to return nb of displayed bytes

perf tools: Change perf_mem__lvl_scnprintf to return nb of displayed bytes

Moving strncat/strcpy calls into scnprintf to easily track number of
displayed bytes. It will be used in following patch.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1456303616-26926-11-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>