percpu_ref: restructure operation mode switching

Restructure atomic/percpu mode switching.

* The users of __percpu_ref_switch_to_atomic/percpu() now call a new
  function __percpu_ref_switch_mode() which calls either of the
  original switching functions depending on the current state of
  ref->force_atomic and the __PERCPU_REF_DEAD flag.  The callers no
  longer check whether switching is necessary but always invoke
  __percpu_ref_switch_mode().

* !ref->confirm_switch waiting is collected into
  __percpu_ref_switch_mode().

This patch doesn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>