Newer kernel versions require this flag to be present contrary to older
ones. Depending on the libnl version it is added or not.

Typically when using rtnl_link_inet6_set_addr_gen_mode, the netlink
packet generated may contain the following attribute:

with libnl 3.4

  {nla_len=16, nla_type=IFLA_AF_SPEC},
  [
    {nla_len=12, nla_type=AF_INET6},
    [{nla_len=5, nla_type=IFLA_INET6_ADDR_GEN_MODE}, IN6_ADDR_GEN_MODE_NONE]
  ]

with libnl 3.7

  {nla_len=16, nla_type=NLA_F_NESTED|IFLA_AF_SPEC},
  [
    {nla_len=12, nla_type=NLA_F_NESTED|AF_INET6},
    [{nla_len=5, nla_type=IFLA_INET6_ADDR_GEN_MODE}, IN6_ADDR_GEN_MODE_NONE]]
  ]

Masking the type is likely needed in other places. Only the above cases
are implemented in this patch.

Signed-off-by: Mathis Marion <mathis.marion@silabs.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20230307154256.101528-3-Mathis.Marion@silabs.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>