rcu: Switch force_qs_rnp() to for_each_leaf_node_cpu_mask()

Currently, force_qs_rnp() uses a for_each_leaf_node_possible_cpu()
loop containing a check of the current CPU's bit in ->qsmask.
This works, but this commit saves three lines by instead using
for_each_leaf_node_cpu_mask(), which combines the functionality of
for_each_leaf_node_possible_cpu() and leaf_node_cpu_bit().  This commit
also replaces the use of the local variable "bit" with rdp->grpmask.

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