rcu: Fix missing task information during rcu-preempt stall

The first item list_for_each_entry_continue(alist) iterates over is
alist->next, rather than alist itself. Consequently,
rcu_print_detail_task_stall_rnp() skips the task referenced by gp_tasks.

Use gp_tasks->prev as the argument to list_for_each_entry_continue()
instead.

Change-Id: Iaa2be14139d0f6c1a1ed25817f0e5737b3e74753
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>