ipv6: preserve insertion order for same-scope addresses

ipv6: preserve insertion order for same-scope addresses

IPv6 addresses with the same scope are returned in reverse insertion
order, unlike IPv4. For example, when adding a -> b -> c, the list is
reported as c -> b -> a, while IPv4 preserves the original order.

This behavior causes:

a. When using `ip -6 a save` and `ip -6 a restore`, addresses are restored
   in the opposite order from which they were saved. See example below
   showing addresses added as 1::1, 1::2, 1::3 but displayed and saved
   in reverse order.

   # ip -6 a a 1::1 dev x
   # ip -6 a a 1::2 dev x
   # ip -6 a a 1::3 dev x
   # ip -6 a s dev x
   2: x: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
       inet6 1::3/128 scope global tentative
       valid_lft forever preferred_lft forever
       inet6 1::2/128 scope global tentative
       valid_lft forever preferred_lft forever
       inet6 1::1/128 scope global tentative
       valid_lft forever preferred_lft forever
   # ip -6 a save > dump
   # ip -6 a d 1::1 dev x
   # ip -6 a d 1::2 dev x
   # ip -6 a d 1::3 dev x
   # ip a d ::1 dev lo
   # ip a restore < dump
   # ip -6 a s dev x
   2: x: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
       inet6 1::1/128 scope global tentative
       valid_lft forever preferred_lft forever
       inet6 1::2/128 scope global tentative
       valid_lft forever preferred_lft forever
       inet6 1::3/128 scope global tentative
       valid_lft forever preferred_lft forever
   # ip a showdump < dump
    if1:
        inet6 ::1/128 scope host proto kernel_lo
        valid_lft forever preferred_lft forever
    if2:
        inet6 1::3/128 scope global tentative
        valid_lft forever preferred_lft forever
    if2:
        inet6 1::2/128 scope global tentative
        valid_lft forever preferred_lft forever
    if2:
        inet6 1::1/128 scope global tentative
        valid_lft forever preferred_lft forever

b. Addresses in pasta to appear in reversed order compared to host
   addresses.

The ipv6 addresses were added in reverse order by commit e55ffac60117
("[IPV6]: order addresses by scope"), then it was changed by commit
502a2ffd7376 ("ipv6: convert idev_list to list macros"), and restored by
commit b54c9b98bbfb ("ipv6: Preserve pervious behavior in
ipv6_link_dev_addr()."). However, this reverse ordering within the same
scope causes inconsistency with IPv4 and the issues described above.

This patch aligns IPv6 address ordering with IPv4 for consistency
by changing the comparison from >= to > when inserting addresses
into the address list. Also updates the ioam6 selftest to reflect
the new address ordering behavior. Combine these two changes into
one patch for bisectability.

Link: https://bugs.passt.top/show_bug.cgi?id=175
Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Yumei Huang <yuhuang@redhat.com>
Acked-by: Justin Iurman <justin.iurman@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260104032357.38555-1-yuhuang@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>