rcu: Do a single rhp->func read in rcu_head_after_call_rcu()

The rcu_head_after_call_rcu() function reads the rhp->func pointer twice,
which can result in a false-positive WARN_ON_ONCE() if the callback
were passed to call_rcu() between the two reads.  Although racing
rcu_head_after_call_rcu() with call_rcu() is to be a dubious use case
(the return value is not reliable in that case), intermittent and
irreproducible warnings are also quite dubious.  This commit therefore
uses a single READ_ONCE() to pick up the value of rhp->func once, then
tests that value twice, thus guaranteeing consistent processing within
rcu_head_after_call_rcu()().

Neverthless, racing rcu_head_after_call_rcu() with call_rcu() is still
a dubious use case.

Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
[ paulmck: Add blank line after declaration per checkpatch.pl. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>