rcu: Fix some kernel-doc warnings

The current code provokes some kernel-doc warnings:

	./kernel/rcu/tree.c:2915: warning: Function parameter or member 'count' not described in 'kfree_rcu_cpu'
	./include/linux/rculist.h:517: warning: bad line:                           [@right ][node2 ... ]
	./include/linux/rculist.h:2: WARNING: Unexpected indentation.

This commit therefore moves the comment for "count" to the kernel-doc
markup and adds a missing "*" on one kernel-doc continuation line.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>