ipvs: don't show negative times in ip_vs_conn

Since commit 500462a9 ("timers: Switch to a non-cascading wheel"),
timers duration can last even 12.5% more than the scheduled interval.

IPVS has two handlers, /proc/net/ip_vs_conn and /proc/net/ip_vs_conn_sync,
which shows the remaining time before that a connection expires.
The default expire time for a connection is 60 seconds, and the
expiration timer can fire even 4 seconds later than the scheduled time.
The expiration time is calculated subtracting jiffies to the scheduled
expiration time, and it's shown as a huge number when the timer fires late,
since both values are unsigned.

This can confuse script and tools which relies on it, like ipvsadm:

    root@mcroce-redhat:~# while ipvsadm -lc |grep SYN_RECV; do sleep 1 ; done
    TCP 00:05  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 00:04  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 00:03  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 00:02  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 00:01  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 00:00  SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:44 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:43 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:42 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:41 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:40 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000
    TCP 68719476:39 SYN_RECV    [fc00:1::1]:55732  [fc00:1::2]:8000   [fc00:2000::1]:8000

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>