xfrm_user: fix return value from xfrm_user_rcv_msg

It doesn't support to run 32bit 'ip' to set xfrm objdect on 64bit host.
But the return value is unknown for user program:

ip xfrm policy list
RTNETLINK answers: Unknown error 524

Replace ENOTSUPP with EOPNOTSUPP:

ip xfrm policy list
RTNETLINK answers: Operation not supported

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>