percpu_ref_init(): clean ->percpu_count_ref on failure

That way percpu_ref_exit() is safe after failing percpu_ref_init().
At least one user (cgroup_create()) had a double-free that way;
there might be other similar bugs.  Easier to fix in percpu_ref_init(),
rather than playing whack-a-mole in sloppy users...

Usual symptoms look like a messed refcounting in one of subsystems
that use percpu allocations (might be percpu-refcount, might be
something else).  Having refcounts for two different objects share
memory is Not Nice(tm)...

Reported-by:  <syzbot+5b1e53987f858500ec00@syzkaller.appspotmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>