xfrm: fix the if_id check in changelink

commit 6d0d95a1c2b07270870e7be16575c513c29af3f1 upstream.

if_id will be always 0, because it was not yet initialized.

Fixes: 8dce43919566 ("xfrm: interface with if_id 0 should return error")
Reported-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>