geneve: allow changing DF behavior after creation

Currently, trying to change the DF parameter of a geneve device does
nothing:

    # ip -d link show geneve1
    14: geneve1: <snip>
        link/ether <snip>
        geneve id 1 remote 10.0.0.1 ttl auto df set dstport 6081 <snip>
    # ip link set geneve1 type geneve id 1 df unset
    # ip -d link show geneve1
    14: geneve1: <snip>
        link/ether <snip>
        geneve id 1 remote 10.0.0.1 ttl auto df set dstport 6081 <snip>

We just need to update the value in geneve_changelink.

Fixes: a025fb5f ("geneve: Allow configuration of DF behaviour")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>