ipv6 addrconf: fix bug where deleting a mngtmpaddr can create a new temporary address

ipv6 addrconf: fix bug where deleting a mngtmpaddr can create a new temporary address

currently on 6.4 net/main:

  # ip link add dummy1 type dummy
  # echo 1 > /proc/sys/net/ipv6/conf/dummy1/use_tempaddr
  # ip link set dummy1 up
  # ip -6 addr add 2000::1/64 mngtmpaddr dev dummy1
  # ip -6 addr show dev dummy1

  11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
      inet6 2000::44f3:581c:8ca:3983/64 scope global temporary dynamic
         valid_lft 604800sec preferred_lft 86172sec
      inet6 2000::1/64 scope global mngtmpaddr
         valid_lft forever preferred_lft forever
      inet6 fe80::e8a8:a6ff:fed5:56d4/64 scope link
         valid_lft forever preferred_lft forever

  # ip -6 addr del 2000::44f3:581c:8ca:3983/64 dev dummy1

  (can wait a few seconds if you want to, the above delete isn't [directly] the problem)

  # ip -6 addr show dev dummy1

  11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
      inet6 2000::1/64 scope global mngtmpaddr
         valid_lft forever preferred_lft forever
      inet6 fe80::e8a8:a6ff:fed5:56d4/64 scope link
         valid_lft forever preferred_lft forever

  # ip -6 addr del 2000::1/64 mngtmpaddr dev dummy1
  # ip -6 addr show dev dummy1

  11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
      inet6 2000::81c9:56b7:f51a:b98f/64 scope global temporary dynamic
         valid_lft 604797sec preferred_lft 86169sec
      inet6 fe80::e8a8:a6ff:fed5:56d4/64 scope link
         valid_lft forever preferred_lft forever

This patch prevents this new 'global temporary dynamic' address from being
created by the deletion of the related (same subnet prefix) 'mngtmpaddr'
(which is triggered by there already being no temporary addresses).

Cc: Jiri Pirko <jiri@resnulli.us>
Fixes: 53bd67491537 ("ipv6 addrconf: introduce IFA_F_MANAGETEMPADDR to tell kernel to manage temporary addresses")
Reported-by: Xiao Ma <xiaom@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230720160022.1887942-1-maze@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>