rtnetlink: add RTEXT_FILTER_NAME_ONLY support to rtnl_dump_ifinfo()

When user requests RTEXT_FILTER_NAME_ONLY flag, we limit the dump
parts to:

 - struct nlmsghdr
 - IFLA_IFNAME
 - IFLA_PROP_LIST (alternate names)

- This saves space in the dump, pushing more devices per system call.
- This can be done without acquiring RTNL.

I still have a medium term goal to avoid RTNL in rtnl_dump_ifinfo()
regardless of RTEXT_FILTER_NAME_ONLY being used.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260525083542.1565964-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>