net: fix two sparse errors

First one in __skb_checksum_validate_complete() fixes the following
(and other callers)

make C=2 CF=-D__CHECK_ENDIAN__ net/ipv4/tcp_ipv4.o
  CHECK   net/ipv4/tcp_ipv4.c
include/linux/skbuff.h:3052:24: warning: incorrect type in return expression (different base types)
include/linux/skbuff.h:3052:24:    expected restricted __sum16
include/linux/skbuff.h:3052:24:    got int

Second is fixing gso_make_checksum() :

  CHECK   net/ipv4/gre_offload.c
include/linux/skbuff.h:3360:14: warning: incorrect type in assignment (different base types)
include/linux/skbuff.h:3360:14:    expected unsigned short [unsigned] [usertype] csum
include/linux/skbuff.h:3360:14:    got restricted __sum16
include/linux/skbuff.h:3365:16: warning: incorrect type in return expression (different base types)
include/linux/skbuff.h:3365:16:    expected restricted __sum16
include/linux/skbuff.h:3365:16:    got unsigned short [unsigned] [usertype] csum

Fixes: 5a21232 ("net: Support for csum_bad in skbuff")
Fixes: 7e2b10c ("net: Support for multiple checksums with gso")
Signed-off-by: Eric Dumazet <edumazet@google.com>
CC: Tom Herbert <tom@herbertland.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3051,7 +3051,7 @@ static inline __sum16 __skb_checksum_validate_complete(struct sk_buff *skb,
 		}
 	} else if (skb->csum_bad) {
 		/* ip_summed == CHECKSUM_NONE in this case */
-		return 1;
+		return (__force __sum16)1;
 	}
 
 	skb->csum = psum;
@@ -3353,15 +3353,14 @@ static inline int gso_pskb_expand_head(struct sk_buff *skb, int extra)
 static inline __sum16 gso_make_checksum(struct sk_buff *skb, __wsum res)
 {
 	int plen = SKB_GSO_CB(skb)->csum_start - skb_headroom(skb) -
-	    skb_transport_offset(skb);
-	__u16 csum;
+		   skb_transport_offset(skb);
+	__wsum partial;
 
-	csum = csum_fold(csum_partial(skb_transport_header(skb),
-				      plen, skb->csum));
+	partial = csum_partial(skb_transport_header(skb), plen, skb->csum);
 	skb->csum = res;
 	SKB_GSO_CB(skb)->csum_start -= plen;
 
-	return csum;
+	return csum_fold(partial);
 }
 
 static inline bool skb_is_gso(const struct sk_buff *skb)