macvlan: fix leak in macvlan_handle_frame

Reset pskb in macvlan_handle_frame in case skb_share_check returned a
clone.

Fixes: 8a4eb573

 ("net: introduce rx_handler results and logic around that")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>