rcu: add DEBUG_OBJECTS_RCU_HEAD check for alignment

Verify that rcu_head structures are aligned to a four-byte boundary.
This check is enabled by CONFIG_DEBUG_OBJECTS_RCU_HEAD.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>