bonding: fix rtnl locking merge error

	Looks like I incorrectly merged one of the rtnl lock changes,
so that one function, bonding_show_active_slave, held rtnl but didn't
release it, and another, bonding_store_active_slave, never held rtnl but
did release it.

	Fixed so the first function doesn't mess with rtnl, and the
second correctly acquires and releases rtnl.

	Bug reported by Moni Shoua <monis@voltaire.com>

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>