rcu: Move rcu_needs_cpu() to tree.c

Now that RCU_FAST_NO_HZ is no more, there is but one implementation of
the rcu_needs_cpu() function.  This commit therefore moves this function
from kernel/rcu/tree_plugin.c to kernel/rcu/tree.c.

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