rcu: Initialize first_gp_fqs at declaration in rcu_gp_fqs()

This commit saves a line of code by initializing the rcu_gp_fqs()
function's first_gp_fqs local variable in its declaration.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>