geneve: fix an uninitialized value in geneve_changelink()

geneve_nl2info() sets 'df' conditionally, so we have to
initialize it by copying the value from existing geneve
device in geneve_changelink().

Fixes: 56c09de3 ("geneve: allow changing DF behavior after creation")
Reported-by:  <syzbot+7ebc2e088af5e4c0c9fa@syzkaller.appspotmail.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>