x25: Do not reference freed memory.

In x25_link_free(), we destroy 'nb' before dereferencing
'nb->dev'.  Don't do this, because 'nb' might be freed
by then.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>