srcu: Remove superfluous callbacks advancing from srcu_gp_start()

Callbacks advancing on SRCU must be performed on two specific places:

1) On enqueue time in order to make room for the acceleration of the
   new callback.

2) On invocation time in order to move the callbacks ready to invoke.

Any other callback advancing callsite is needless. Remove the remaining
one in srcu_gp_start().

Co-developed-by: Yong He <zhuangel570@gmail.com>
Signed-off-by: Yong He <zhuangel570@gmail.com>
Co-developed-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Co-developed-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>