percpu, locking: Revert ("percpu: Replace smp_read_barrier_depends() with lockless_dereference()")

lockless_dereference() is planned to grow a sanity check to ensure
that the input parameter is a pointer.  __ref_is_percpu() passes in an
unsinged long value which is a combination of a pointer and a flag.
While it can be casted to a pointer lvalue, the casting looks messy
and it's a special case anyway.  Let's revert back to open-coding
READ_ONCE() and explicit barrier.

This doesn't cause any functional changes.

Signed-off-by: Tejun Heo <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Paul McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Pranith Kumar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/g/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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