Current runs show PREEMPT=n rcu_read_lock()/rcu_read_unlock() pairs
consuming between 20 and 30 nanoseconds, when in fact the actual value is
zero, give or take the barrier() asm's effect on compiler optimizations.
The additional overhead is caused by function calls through pointers
(especially in these days of Spectre mitigations) and perhaps also
needless argument passing, a non-const loop limit, and an upcounting loop.

This commit therefore combines the ->readlock() and ->readunlock()
function pointers into a single ->readsection() function pointer that
takes the loop count as a const parameter and keeps any data passed
from the read-lock to the read-unlock internal to this new function.

These changes reduce the measured overhead of the aforementioned
PREEMPT=n rcu_read_lock()/rcu_read_unlock() pairs from between 20 and
30 nanoseconds to somewhere south of 500 picoseconds.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>