xen: avoid allocation causing potential swap activity on the resume path

Since the device we are resuming could be the device containing the
swap device we should ensure that the allocation cannot cause
IO.

On resume, this path is triggered when the running system tries to
continue using its devices.  If it cannot then the resume will fail;
to try to avoid this we let it dip into the emergency pools.

The majority of these changes were made when linux-2.6.18-xen.hg
changeset e8b49cfbdac0 was ported upstream in
a144ff09 but somehow this hunk was
dropped.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Stable Kernel <stable@kernel.org> # .32.x