ipv6: fix omitted netlink attributes when using RTEXT_FILTER_SKIP_STATS

Using RTEXT_FILTER_SKIP_STATS is incorrectly skipping non-stats IPv6
netlink attributes on link dump. This causes issues on userspace tools,
e.g iproute2 is not rendering address generation mode as it should due
to missing netlink attribute.

Move the filling of IFLA_INET6_STATS and IFLA_INET6_ICMP6STATS to a
helper function guarded by a flag check to avoid hitting the same
situation in the future.

Fixes: d5566fd7 ("rtnetlink: RTEXT_FILTER_SKIP_STATS support to avoid dumping inet/inet6 stats")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250402121751.3108-1-ffmancera@riseup.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>