ip-monitor: Do not error out when RTNLGRP_STATS is not available

ip-monitor: Do not error out when RTNLGRP_STATS is not available

Following commit 4e8a9914c4d4 ("ip-monitor: Include stats events in default
and "all" cases"), `ip monitor` fails to start on kernels which do not
contain linux.git commit 5fd0b838efac ("net: rtnetlink: Add UAPI toggle for
IFLA_OFFLOAD_XSTATS_L3_STATS") because the netlink group RTNLGRP_STATS
doesn't exist:

 $ ip monitor
 Failed to add stats group to list

When "stats" is not explicitly requested, ignore the error so that `ip
monitor` and `ip monitor all` continue to work on older kernels.

Note that the same change is not done for RTNLGRP_NEXTHOP because its value
is 32 and group numbers <= 32 are always supported; see the comment above
netlink_change_ngroups() in the kernel source. Therefore
NETLINK_ADD_MEMBERSHIP 32 does not error out even on kernels which do not
support RTNLGRP_NEXTHOP.

v2:
* Silently ignore a failure to implicitly add the stats group, instead of
  printing a warning.

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Fixes: 4e8a9914c4d4 ("ip-monitor: Include stats events in default and "all" cases")
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>