This commit adds an rcu_pointer_handoff() that is intended to mark
situations where a structure's protection transitions from RCU to some
other mechanism (locking, reference counting, whatever).  These markings
should allow external tools to more easily spot bugs involving leaking
pointers out of RCU read-side critical sections.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>