net: inet_diag -- Return error code if protocol handler is missed

net: inet_diag -- Return error code if protocol handler is missed

We've observed that in case if UDP diag module is not
supported in kernel the netlink returns NLMSG_DONE without
notifying a caller that handler is missed.

This patch makes __inet_diag_dump to return error code instead.

So as example it become possible to detect such situation
and handle it gracefully on userspace level.

Signed-off-by: Cyrill Gorcunov <[email protected]>
CC: David Miller <[email protected]>
CC: Eric Dumazet <[email protected]>
CC: Pavel Emelyanov <[email protected]>
Acked-by: Pavel Emelyanov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>