xfrm_user: fix return value from xfrm_user_rcv_msg

xfrm_user: fix return value from xfrm_user_rcv_msg

commit 83e2d0587ae859aae75fd9d246c409b10a6bd137 upstream.

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>
Signed-off-by: Nathan Harold <nharold@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>