xen/arm: add_to_physmap_one: Avoid to map mfn 0 if an error occurs

xen/arm: add_to_physmap_one: Avoid to map mfn 0 if an error occurs

By default, the function add_to_physmap_one set mfn to 0. Some code paths that
result to an error, continue and the map the mfn 0 (valid on ARM) to the
slot given by the guest.

To fix the problem, return directly an error if sanity check has failed.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>