dhcpv6-ia: Check lockf return value

dhcpv6-ia: Check lockf return value

Check the lockf return value to fix the following warning/error:

/home/fainelli/dev/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/odhcpd-2017-02-21-1b630f8b/src/dhcpv6-ia.c: In function 'dhcpv6_write_statefile':
/home/fainelli/dev/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/odhcpd-2017-02-21-1b630f8b/src/dhcpv6-ia.c:246:8: error: ignoring return value of 'lockf', declared with attribute warn_unused_result [-Werror=unused-result]
   lockf(fd, F_LOCK, 0);
        ^
cc1: all warnings being treated as errors

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>