devlink: mnlg: Catch returned error value of dumpit commands

devlink: mnlg: Catch returned error value of dumpit commands

Devlink commands which implements the dumpit callback may return error.
The netlink function netlink_dump() sends the errno value as the payload
of the message, while answering user space with NLMSG_DONE.
To enable receiving errno value for dumpit commands we have to check for
it in the message. If it is a negative value then the dump returned an
error so we should set errno accordingly and check for ext_ack in case
it was set.

Fixes: 049c58539f5d ("devlink: mnlg: Add support for extended ack")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>