hash: Fix hash seed conditional

Fix the marshall of the hash seed to be conditional, only if it is
explicitly set, we need to add it to the kernel as stated on the
libnftl and nftables projects.
Refence: https://git.netfilter.org/nftables/tree/src/netlink_linearize.c#n174

Otherwise, having a hash expression similar to this:
```
ip daddr set jhash tcp sport mod 2 seed 0x0 map { 0 : 192.168.0.1, 1 : 192.168.2.2 }
```
end up setting only the first IP and ignoring the second one.

Signed-off-by: Rafael Campos <methril@gmail.com>

expr/hash.go

@@ -41,15 +41,22 @@ type Hash struct {
 }
 
 func (e *Hash) marshal(fam byte) ([]byte, error) {
-	data, err := netlink.MarshalAttributes([]netlink.Attribute{
+	hashAttrs := []netlink.Attribute{
 		{Type: unix.NFTA_HASH_SREG, Data: binaryutil.BigEndian.PutUint32(uint32(e.SourceRegister))},
 		{Type: unix.NFTA_HASH_DREG, Data: binaryutil.BigEndian.PutUint32(uint32(e.DestRegister))},
 		{Type: unix.NFTA_HASH_LEN, Data: binaryutil.BigEndian.PutUint32(uint32(e.Length))},
 		{Type: unix.NFTA_HASH_MODULUS, Data: binaryutil.BigEndian.PutUint32(uint32(e.Modulus))},
-		{Type: unix.NFTA_HASH_SEED, Data: binaryutil.BigEndian.PutUint32(uint32(e.Seed))},
-		{Type: unix.NFTA_HASH_OFFSET, Data: binaryutil.BigEndian.PutUint32(uint32(e.Offset))},
+	}
+	if e.Seed != 0 {
+		hashAttrs = append(hashAttrs, netlink.Attribute{
+			Type: unix.NFTA_HASH_SEED, Data: binaryutil.BigEndian.PutUint32(uint32(e.Seed)),
+		})
+	}
+	hashAttrs = append(hashAttrs, []netlink.Attribute{
 		{Type: unix.NFTA_HASH_TYPE, Data: binaryutil.BigEndian.PutUint32(uint32(e.Type))},
-	})
+		{Type: unix.NFTA_HASH_OFFSET, Data: binaryutil.BigEndian.PutUint32(uint32(e.Offset))},
+	}...)
+	data, err := netlink.MarshalAttributes(hashAttrs)
 	if err != nil {
 		return nil, err
 	}