srcu: Implement more-efficient reader counts

SRCU uses two per-cpu counters: a nesting counter to count the number of
active critical sections, and a sequence counter to ensure that the nesting
counters don't change while they are being added together in
srcu_readers_active_idx_check().

This patch instead uses per-cpu lock and unlock counters. Because both
counters only increase and srcu_readers_active_idx_check() reads the unlock
counter before the lock counter, this achieves the same end without having
to increment two different counters in srcu_read_lock(). This also saves a
smp_mb() in srcu_readers_active_idx_check().

Possible bug: There is no guarantee that the lock counter won't overflow
during srcu_readers_active_idx_check(), as there are no memory barriers
around srcu_flip() (see comment in srcu_readers_active_idx_check() for
details). However, this problem was already present before this patch.

Suggested-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: Lance Roy <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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