zdtm/net/ipv4: split save_and_set and check_and_restore

As changing disable_ipv6 sysctl for some device may change mtu sysctl
for it we need to first check mtu, and only then set disable_ipv6.
That can be done splitting our functions into two.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>