percpu: speed alloc_pcpu_area() up

If we know that first N areas are all in use, we can obviously skip
them when searching for a free one.  And that kind of hint is very
easy to maintain.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --cc mm/percpu.c
Simple merge