[XFRM] STATE: Fix to respond error to get operation if no matching entry exists.

[XFRM] STATE: Fix to respond error to get operation if no matching entry exists.

When application uses XFRM_MSG_GETSA to get state entry through
netlink socket and kernel has no matching one, the application expects
reply message with error status by kernel.

Kernel doesn't send the message back in the case of Mobile IPv6 route
optimization protocols (i.e. routing header or destination options
header). This is caused by incorrect return code "0" from
net/xfrm/xfrm_user.c(xfrm_user_state_lookup) and it makes kernel skip
to acknowledge at net/netlink/af_netlink.c(netlink_rcv_skb).

This patch fix to reply ESRCH to application.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: TAKAMIYA Noriaki <takamiya@po.ntts.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>