datapath: Don't call genlmsg_reply() under rcu_read_lock().
genlmsg_reply() indirectly makes a call to kmalloc but takes no
GFP flags, instead using GFP_ATOMIC if in a softirq and GFP_KERNEL
otherwise. However, here we hold rcu_read_lock(), which requires
GFP_ATOMIC but is not a softirq. Since we've already built the
reply message, it is safe to release rcu_read_lock(), so do that
before calling genlmsg_reply().