zdtm/net/ipv4: split save_and_set and check_and_restore
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 <3e8c75167f9f00ca74287a4609f1507a2696e07e@virtuozzo.com>
Reviewed-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,KKoukiou/criu-remote,efiop/criu,KKoukiou/criu-remote,eabatalov/criu,efiop/criu,efiop/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,KKoukiou/criu-remote,efiop/criu,efiop/criu