rcu: Replace the rcu_barrier enum with pointer to call_rcu*() function

rcu: Replace the rcu_barrier enum with pointer to call_rcu*() function

The rcu_barrier enum causes several problems:

  (1) you have to define the enum somewhere, and there is no
      convenient place,

  (2) the difference between TREE_RCU and TREE_PREEMPT_RCU causes
      problems when you need to map from rcu_barrier enum to struct
      rcu_state,

  (3) the switch statement are large, and

  (4) TINY_RCU really needs a different rcu_barrier() than do the
      treercu implementations.

So replace it with a functionally equivalent but cleaner function
pointer abstraction.

Signed-off-by: Paul E. McKenney <[email protected]>
Acked-by: Mathieu Desnoyers <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
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LKML-Reference: <12541998232366-git-send-email->
Signed-off-by: Ingo Molnar <[email protected]>