perf tools: Avoid unnecessary work in directory lookups

This patch improves some (common) inefficiencies in the
handling of directory lookups:

- not using the d_type information returned by the kernel

- constructing (absolute) paths for file operation even though
  directory-relative operations using the *at functions is
  possible

There are more places to fix but this is a start.

Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20090904193951.GB6186@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>