net/ipv4: add net_conf_op to reuse for ipv6

use new SysctlEntry, leave old ipv4_conf_op as ipv4_conf_op_old
for forward compatibility

v4: use CTL_FLAGS_HAS instead of req[].has
v5: use CTL_TYPE in sysctl_entries_equal
v6: fix net_conf_op for string sysctls, add rconf to have requests
conf at hand

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