ipv4: validate IPV4_DEVCONF attributes properly

ipv4: validate IPV4_DEVCONF attributes properly

As the IPV4_DEVCONF netlink attributes are not being validated, it is
possible to use netlink to set read-only values like mc_forwarding. In
addition, valid ranges are not being validated neither but that is less
relevant as they aren't in sysctl.

To avoid similar situations in the future, define a NLA policy for
IPV4_DEVCONF attributes which are nested in IFLA_INET_CONF.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260312142637.5704-1-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>