[KEY]: Fix conversion between IPSEC_MODE_xxx and XFRM_MODE_xxx.

We should not blindly convert between IPSEC_MODE_xxx and XFRM_MODE_xxx just
by incrementing / decrementing because the assumption is not true any longer.

Signed-off-by: Kazunori MIYAZAWA <miyazawa@linux-ipv6.org>
Singed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>