selinux: implement the kernfs_init_security hook

The hook applies the same logic as selinux_determine_inode_label(), with
the exception of the super_block handling, which will be enforced on the
actual inodes later by other hooks.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
[PM: minor merge fixes]
Signed-off-by: Paul Moore <paul@paul-moore.com>