perf tools: Modify perf routines to use new debugfs routines

modify perf.c get_debugfs_mntpnt() to use the util/debugfs.c
debugfs_find_mountpoint()

modify util/parse-events.c to use debugfs_valid_mountpoint().

Signed-off-by: Clark Williams <williams@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20091101155720.624cc87e@torg>
Signed-off-by: Ingo Molnar <mingo@elte.hu>