flow_keys: n_proto type should be __be16

(struct flow_keys)->n_proto is in network order, use
proper type for this.

Fixes following sparse errors :

net/core/flow_dissector.c:139:39: warning: incorrect type in assignment (different base types)
net/core/flow_dissector.c:139:39:    expected unsigned short [unsigned] [usertype] n_proto
net/core/flow_dissector.c:139:39:    got restricted __be16 [assigned] [usertype] proto
net/core/flow_dissector.c:237:23: warning: incorrect type in assignment (different base types)
net/core/flow_dissector.c:237:23:    expected unsigned short [unsigned] [usertype] n_proto
net/core/flow_dissector.c:237:23:    got restricted __be16 [assigned] [usertype] proto

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: e0f31d84 ("flow_keys: Record IP layer protocol in skb_flow_dissect()")
Signed-off-by: David S. Miller <davem@davemloft.net>