sysctl: add CTL_FLAGS_HAS to mark successful sysctl_op request

v4: replace separate has pointer to CTL_FLAGS_HAS flag, second part in
patch "net/ipv4: add net_conf_op to reuse for ipv6"
v6: define CTL_FLAGS_HAS
v7: also allow EIO on do_sysctl_op for optional sysctls like
stable_secret and fix sysctl file to close in error path
v9: add CTL_FLAGS_READ_EIO_SKIP to skip dumping stable_secret on EIO

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