rcu/nocb: Fix segcblist state machine comments about bypass

The parts explaining the bypass lifecycle in (de-)offloading are out
of date and/or wrong. Bypass is simply enabled whenever SEGCBLIST_RCU_CORE
flag is off.

Fix the comments accordingly.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>