rcu: Fix init_rcu_head() comment.

The current (and implicit) comment header for init_rcu_head() and
destroy_rcu_head() incorrectly says that they are not needed for
statically allocated rcu_head structures.  This commit therefore
fixes this comment.

Reported-by: Bart Van Assche <Bart.VanAssche@wdc.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>