percpu_ref: decouple switching to atomic mode and killing

percpu_ref has treated the dropping of the base reference and
switching to atomic mode as an integral operation; however, there's
nothing inherent tying the two together.

The use cases for percpu_ref have been expanding continuously.  While
the current init/kill/reinit/exit model can cover a lot, the coupling
of kill/reinit with atomic/percpu mode switching is turning out to be
too restrictive for use cases where many percpu_refs are created and
destroyed back-to-back with only some of them reaching extended
operation.  The coupling also makes implementing always-atomic debug
mode difficult.

This patch separates out atomic mode switching into
percpu_ref_switch_to_atomic() and reimplements
percpu_ref_kill_and_confirm() on top of it.

* The handling of __PERCPU_REF_ATOMIC and __PERCPU_REF_DEAD is now
  differentiated.  Among get/put operations, percpu_ref_tryget_live()
  is the only one which cares about DEAD.

* percpu_ref_switch_to_atomic() can be called multiple times on the
  same ref.  This means that multiple @confirm_switch may get queued
  up which we can't do reliably without extra memory area.  This is
  handled by making the later invocation synchronously wait for the
  completion of the previous one.  This isn't particularly desirable
  but such synchronous waits shouldn't happen in most cases.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>