devlink: support kernel-side snapshot id allocation

devlink: support kernel-side snapshot id allocation

Make ID argument optional and read the snapshot info
that kernel sends us.

$ devlink region new netdevsim/netdevsim1/dummy
netdevsim/netdevsim1/dummy: snapshot 0
$ devlink -jp region new netdevsim/netdevsim1/dummy
{
    "regions": {
        "netdevsim/netdevsim1/dummy": {
            "snapshot": [ 1 ]
        }
    }
}
$ devlink region show netdevsim/netdevsim1/dummy
netdevsim/netdevsim1/dummy: size 32768 snapshot [0 1]

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>