netlink: ensure to loop over all netns in genlmsg_multicast_allns()

netlink: ensure to loop over all netns in genlmsg_multicast_allns()

commit cb9f7a9a5c96a773bbc9c70660dc600cfff82f82 upstream.

Nowadays, nlmsg_multicast() returns only 0 or -ESRCH but this was not the
case when commit 134e63756d5f was pushed.
However, there was no reason to stop the loop if a netns does not have
listeners.
Returns -ESRCH only if there was no listeners in all netns.

To avoid having the same problem in the future, I didn't take the
assumption that nlmsg_multicast() returns only 0 or -ESRCH.

Fixes: 134e63756d5f ("genetlink: make netns aware")
CC: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: s/portid/pid/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>