kernfs: initialize security of newly created nodes

Use the new security_kernfs_init_security() hook to allow LSMs to
possibly assign a non-default security context to a newly created kernfs
node based on the attributes of the new node and also its parent node.

This fixes an issue with cgroupfs under SELinux, where newly created
cgroup subdirectories/files would not inherit its parent's context if
it had been set explicitly to a non-default value (other than the genfs
context specified by the policy). This can be reproduced as follows (on
Fedora/RHEL):

    # mkdir /sys/fs/cgroup/unified/test
    # # Need permissive to change the label under Fedora policy:
    # setenforce 0
    # chcon -t container_file_t /sys/fs/cgroup/unified/test
    # ls -lZ /sys/fs/cgroup/unified
    total 0
    -r--r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.controllers
    -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.max.depth
    -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.max.descendants
    -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.procs
    -r--r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.stat
    -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.subtree_control
    -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 cgroup.threads
    drwxr-xr-x.  2 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 init.scope
    drwxr-xr-x. 26 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:21 system.slice
    drwxr-xr-x.  3 root root system_u:object_r:container_file_t:s0 0 Jan 29 03:15 test
    drwxr-xr-x.  3 root root system_u:object_r:cgroup_t:s0         0 Jan 29 03:06 user.slice
    # mkdir /sys/fs/cgroup/unified/test/subdir

Actual result:

    # ls -ldZ /sys/fs/cgroup/unified/test/subdir
    drwxr-xr-x. 2 root root system_u:object_r:cgroup_t:s0 0 Jan 29 03:15 /sys/fs/cgroup/unified/test/subdir

Expected result:

    # ls -ldZ /sys/fs/cgroup/unified/test/subdir
    drwxr-xr-x. 2 root root unconfined_u:object_r:container_file_t:s0 0 Jan 29 03:15 /sys/fs/cgroup/unified/test/subdir

Link: https://github.com/SELinuxProject/selinux-kernel/issues/39

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>