rcu: Fix a kernel-doc warnings for "count"

There are some kernel-doc warnings:

	./kernel/rcu/tree.c:2915: warning: Function parameter or member 'count' not described in 'kfree_rcu_cpu'

This commit therefore moves the comment for "count" to the kernel-doc
markup.

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