percpu_ref: relocate percpu_ref_reinit()

percpu_ref is gonna go through restructuring.  Move
percpu_ref_reinit() after percpu_ref_kill_and_confirm().  This will
make later changes easier to follow and result in cleaner
organization.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Kent Overstreet <kmo@daterainc.com>