perf fs: Rename NAME_find_mountpoint() to NAME__mountpoint()

Shorten it, "finding" it is an implementation detail, what callers want
is the pathname, not to ask for it to _always_ do the lookup.

And the existing implementation already caches it, i.e. it doesn't
"finds" it on every call.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-r24wa4bvtccg7mnkessrbbdj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>