macvlan: fix macvlan_get_size()

commit df8ef8f3 (macvlan: add FDB bridge ops and macvlan flags)
forgot to update macvlan_get_size() after the addition of
IFLA_MACVLAN_FLAGS

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>