netfilter: ipset: timeout can be modified for already added elements

When an element to a set with timeout added, one can change the timeout
by "readding" the element with the "-exist" flag. That means the timeout
value is reset to the specified one (or to the default from the set
specification if the "timeout n" option is not used). Example

ipset add foo 1.2.3.4 timeout 10
ipset add foo 1.2.3.4 timeout 600 -exist

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>