rcu: Test synchronous RCU grace periods at the end of rcu_init()

This commit tests synchronize_rcu() and synchronize_rcu_expedited()
at the end of rcu_init(), in addition to the test already at the
beginning of that function.  These tests are run only in kernels built
with CONFIG_PROVE_RCU=y.

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