arm/xen: Enable user access to the kernel before issuing a privcmd call

arm/xen: Enable user access to the kernel before issuing a privcmd call

When Xen is copying data to/from the guest it will check if the kernel
has the right to do the access. If not, the hypercall will return an
error.

After the commit c0f153321b9e24284261c28ec7dda6d85a0d9b25 "ARM:
software-based privileged-no-access support", the kernel can't access
any longer the user space by default. This will result to fail on every
hypercall made by the userspace (i.e via privcmd).

We have to enable the userspace access and then restore the correct
permission every time the privcmd is used to made an hypercall.

I didn't find generic helpers to do a these operations, so the change
is only arm32 specific.

Reported-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>