rcu: Fix kfree_rcu() docbook errors

After commit 5130b8fd ("rcu: Introduce kfree_rcu() single-argument macro"),
kernel-doc now emits two warnings:

	./include/linux/rcupdate.h:884: warning: Excess function parameter 'ptr' description in 'kfree_rcu'
	./include/linux/rcupdate.h:884: warning: Excess function parameter 'rhf' description in 'kfree_rcu'

This commit added some macro magic in order to call two different versions
of kfree_rcu(), the first having just one argument and the second having
two arguments.  That makes it difficult to document the kfree_rcu() arguments
in the docboook header.

In order to make clearer that this macro accepts optional arguments,
this commit uses macro concatenation so that this macro changes from:
	#define kfree_rcu kvfree_rcu

to:
	#define kfree_rcu(ptr, rhf...) kvfree_rcu(ptr, ## rhf)

That not only helps kernel-doc understand the macro arguments, but also
provides a better C definition that makes clearer that the first argument
is mandatory and the second one is optional.

Fixes: 5130b8fd ("rcu: Introduce kfree_rcu() single-argument macro")
Tested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>