ipmonitor: fix ip monitor can't work when NET_NS is not enabled

ipmonitor: fix ip monitor can't work when NET_NS is not enabled

In ip monitor, netns_map_init will check getnsid is supported or not.
But when /proc/self/ns/net does not exist, we just print out error
messages and exit. So user cannot use ip monitor anymore when
CONFIG_NET_NS is disabled:
  # ip monitor
  open("/proc/self/ns/net"): No such file or directory

If open "/proc/self/ns/net" failed, set have_rtnl_getnsid to false.

Fixes: d652ccbf8195 ("netns: allow to dump and monitor nsid")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>