ipv4: handle devconf post-set actions on netlink updates

ipv4: handle devconf post-set actions on netlink updates

When IPv4 device configuration parameters are updated via netlink, the
kernel currently only updates the value. This bypasses several
post-modification actions that occur when these same parameters are
updated via sysctl, such as flushing the routing cache or emitting
RTM_NEWNETCONF notifications.

This patch addresses the inconsistency by calling the
devinet_conf_post_set() helper inside inet_set_link_af(). If a flush is
required, we defer it until the netlink attribute parsing loop
completes.

This ensures consistent behavior and side-effects for devconf changes,
regardless of whether they are initiated via sysctl or netlink.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260609204520.4670-2-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>