percpu: implement generic percpu refcounting

This implements a refcount with similar semantics to
atomic_get()/atomic_dec_and_test() - but percpu.

It also implements two stage shutdown, as we need it to tear down the
percpu counts.  Before dropping the initial refcount, you must call
percpu_ref_kill(); this puts the refcount in "shutting down mode" and
switches back to a single atomic refcount with the appropriate
barriers (synchronize_rcu()).

It's also legal to call percpu_ref_kill() multiple times - it only
returns true once, so callers don't have to reimplement shutdown
synchronization.

[[email protected]: fix build]
[[email protected]: coding-style tweak]
Signed-off-by: Kent Overstreet <[email protected]>
Cc: Zach Brown <[email protected]>
Cc: Felipe Balbi <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Asai Thambi S P <[email protected]>
Cc: Selvan Mani <[email protected]>
Cc: Sam Bradshaw <[email protected]>
Cc: Jeff Moyer <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Ingo Molnar <[email protected]>
Reviewed-by: "Theodore Ts'o" <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

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