SELINUX: add /sys/fs/selinux mount point to put selinuxfs

In the interest of keeping userspace from having to create new root
filesystems all the time, let's follow the lead of the other in-kernel
filesystems and provide a proper mount point for it in sysfs.

For selinuxfs, this mount point should be in /sys/fs/selinux/

Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Lennart Poettering <mzerqung@0pointer.de>
Cc: Daniel J Walsh <dwalsh@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[include kobject.h - Eric Paris]
[use selinuxfs_obj throughout - Eric Paris]
Signed-off-by: Eric Paris <eparis@redhat.com>