xenfs: Use proc_create_mount_point() to create /proc/xen

xenfs: Use proc_create_mount_point() to create /proc/xen

Mounting proc in user namespace containers fails if the xenbus
filesystem is mounted on /proc/xen because this directory fails
the "permanently empty" test. proc_create_mount_point() exists
specifically to create such mountpoints in proc but is currently
proc-internal. Export this interface to modules, then use it in
xenbus when creating /proc/xen.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
OraBug: 25497392

(cherry picked from commit f97df70b1c879f764f88b25b0e67b03a5213968a)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>