netlink: add nlmsg_consume() and use it in devlink compat

netlink: add nlmsg_consume() and use it in devlink compat

devlink_compat_running_version() sticks out when running
netdevsim tests and watching dropped skbs. Add nlmsg_consume()
for cases were we want to free a netlink skb but it is expected,
rather than a drop. af_netlink code uses consume_skb() directly,
which is fine, but some may prefer the symmetry of nlmsg_new() /
nlmsg_consume().

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>