rcu: Inline rcu_dynticks_momentary_idle() into its sole caller

The rcu_dynticks_momentary_idle() function is invoked only from
rcu_momentary_dyntick_idle(), and neither function is particularly
large.  This commit therefore saves a few lines by inlining
rcu_dynticks_momentary_idle() into rcu_momentary_dyntick_idle().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>