driver: ipvlan: Remove unnecessary ipvlan NULL check in ipvlan_count_rx

There are three functions which would invoke the ipvlan_count_rx. They
are ipvlan_process_multicast, ipvlan_rcv_frame, and ipvlan_nf_input.
The former two functions already use the ipvlan directly before
ipvlan_count_rx, and ipvlan_nf_input gets the ipvlan from
ipvl_addr->master, it is not possible to be NULL too.
So the ipvlan pointer check is unnecessary in ipvlan_count_rx.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>