sysctl: append '\0' in sysctl_read_char

Before these patch one need to zero-init char buffer before giving
it to sysctl_op, it can be convenient to remove these restriction.

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