rcu: Fix description of kvfree_rcu()

The kvfree_rcu() header comment's description of the "ptr" parameter
is unclear, therefore rephrase it to make it clear that it is a pointer
to the memory to eventually be passed to kvfree().

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>