perf trace: Handle DT_UNKNOWN on filesystems that don't support d_type

Some filesystems like xfs and reiserfs will return DT_UNKNOWN for the
d_type.  Handle this case by calling stat() to determine the type.

Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1290355779-3276-1-git-send-email-sbohrer@rgmadvisors.com>
Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>