rcu: Update rcu_bind_gp_kthread() header comment

The header comment for rcu_bind_gp_kthread() refers to sysidle, which
is no longer with us.  However, it is still important to bind RCU's
grace-period kthreads to the housekeeping CPU(s), so rather than remove
rcu_bind_gp_kthread(), this commit updates the comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>