ipvs: Improve robustness to the ipvs sysctl

commit 1b90af292e71b20d03b837d39406acfbdc5d4b2a upstream.

The ipvs module parse the user buffer and save it to sysctl,
then check if the value is valid. invalid value occurs
over a period of time.
Here, I add a variable, struct ctl_table tmp, used to read
the value from the user buffer, and save only when it is valid.
I delete proc_do_sync_mode and use extra1/2 in table for the
proc_dointvec_minmax call.

Fixes: f73181c8 ("ipvs: add support for sync threads")
Signed-off-by: Junwei Hu <hujunwei4@huawei.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[Julian: Backport by changing SYSCTL_ZERO/SYSCTL_ONE to zero/one]
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>