xfrm: fix the return code when xfrm_*_register_afinfo failed

If xfrm_*_register_afinfo failed since xfrm_*_afinfo[afinfo->family] had the
value, return the -EEXIST, not -ENOBUFS

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

diff --cc net/xfrm/xfrm_input.c
Simple merge

diff --cc net/xfrm/xfrm_policy.c
Simple merge

diff --cc net/xfrm/xfrm_state.c
Simple merge