x86: check paging mode earlier in xenmem_add_to_physmap_one()

x86: check paging mode earlier in xenmem_add_to_physmap_one()

There's no point in deferring this until after some initial processing,
and it's actively wrong for the XENMAPSPACE_gmfn_foreign handling to not
have such a check at all.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: f38f3dccf1e1a8aabcf57364326fc8f44cddc41a
master date: 2017-11-28 13:14:43 +0100