rcu: Move __rcu_read_lock() and __rcu_read_unlock() to per-CPU variables

rcu: Move __rcu_read_lock() and __rcu_read_unlock() to per-CPU variables

This commit is another step towards inlinable __rcu_read_lock() and
__rcu_read_unlock() functions for preemptible RCU.  This keeps these two
functions out of line, but switches them to use the per-CPU variables
that are required to export their definitions without requiring that
all RCU users include sched.h.  These per-CPU variables are saved and
restored at context-switch time.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>