[PATCH] ppc64: fix up()/down() usage for kprobe_mutex

The incorrect kprobe_mutex usage on x86_64 had percolated to ppc64 too.
First noticed by Yanmin Zhang.

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>