zdtm/netns-dev: restrict hop_limit sysctl setting to range [1;255]

$ git log v4.4-rc1-465-gd6df198
commit d6df198d924775e4751561cf60ef0294e95f74df
Author: Phil Sutter <phil@nwl.cc>
Date:   Tue Dec 1 22:45:15 2015 +0100

    net: ipv6: restrict hop_limit sysctl setting to range [1; 255]

    Setting a value bigger than 255 resulted in using only the lower eight
    bits of that value as it is assigned to the u8 header field. To avoid
    this unexpected result, reject such values.

https://github.com/xemul/criu/issues/164


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