bonding: Use RCU_INIT_POINTER() for better overhead and for sparse

Although rcu_assign_pointer() can be used to assign a constant
NULL pointer, doing so gets you an unnecessary memory barrier and
in some circumstances, sparse warnings.  This commit therefore
changes the rcu_assign_pointer() of NULL in __bond_release_one() to
RCU_INIT_POINTER().

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