rcu: Add state name to show_rcu_gp_kthreads() output

This commit adds the name of the RCU grace-period state to
the show_rcu_gp_kthreads() output in order to ease debugging.
This commit also moves gp_state_getname() up in the code so that
show_rcu_gp_kthreads() can use it.

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