mm/page_owner.c: use late_initcall to hook in enabling

This was using module_init, but there is no way this code can
be modular.  In the non-modular case, a module_init becomes a
device_initcall, but this really isn't a device.   So we should
choose a more appropriate initcall bucket to put it in.

In order of execution, our close choices are:

 fs_initcall(fn)
 rootfs_initcall(fn)
 device_initcall(fn)
 late_initcall(fn)

..and since the initcall here goes after debugfs, we really
should be post-rootfs, which means late_initcall makes the
most sense here.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>