genl_add_mcast_grp doesn't set errno in all cases.
On kernels that support mptcp but lack event support (all kernels <= 5.11)
MPTCP_PM_EV_GRP_NAME won't be found and ip will exit with
"can't subscribe to mptcp events: Success"
Set errno to a meaningful value (ENOENT) when the group name isn't found
and also cover other spots where it returns nonzero with errno unset.
Fixes: ff619e4fd3 ("mptcp: add support for event monitoring")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
d3740fdc26