sysctl: always initialize i_uid/i_gid

In the Linux kernel, the following vulnerability has been resolved: sysctl: always initialize i_uid/i_gid Always initialize i_uid/i_gid inside the sysfs core so set_ownership() can safely skip setting them. Commit 5ec27ec735ba ("fs/proc/proc_sysctl.c: fix the default values of i_uid/i_gid on /proc/sys inodes.") added defaults for i_uid/i_gid when set_ownership() was not implemented. It also missed adjusting net_ctl_set_ownership() to use the same default values in case the computation of a better value failed.