[NETFILTER]: nf_conntrack: fix ct_extend ->move operation

The ->move operation has two bugs:

- It is called with the same extension as source and destination,
  so it doesn't update the new extension.

- The address of the old extension is calculated incorrectly,
  instead of (void *)ct->ext + ct->ext->offset[i] it uses
  ct->ext + ct->ext->offset[i].

Fixes a crash on x86_64 reported by Chuck Ebbert <cebbert@redhat.com>
and Thomas Woerner <twoerner@redhat.com>.

Tested-by: Thomas Woerner <twoerner@redhat.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>