ll_map: add RTEXT_FILTER_NAME_ONLY to ll_link_get() and ll_init_map()

iproute2 can spend considerable amount of time in ll_init_map()
or ll_link_get() to dump verbose netdev attributes, contributing
to RTNL pressure.

Add RTEXT_FILTER_NAME_ONLY new flag so that rtnl_fill_ifinfo()
limits its output to:

    - struct nlmsghdr
    - IFLA_IFNAME
    - IFLA_PROP_LIST (alternate names)

This is supported in linux 7.2+ (and ignored in old kernels).

RTNL avoidance should be automatically provided in upcoming
kernel patches.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David Ahern <dsahern@kernel.org>