x86/p2m-ept: don't unmap the EPT pagetable while it is still in use

The call to iommu_pte_flush() between the two hunks uses &ept_entry->epte
which is a pointer into the mapped page.

It is eventually passed to `clflush` instruction which will suffer a pagefault
if the virtual mapping has fallen out of the TLB.

    (XEN) ----[ Xen-4.5.0-xs102594-d  x86_64  debug=y  Not tainted ]----
    (XEN) CPU:    7
    (XEN) RIP:    e008:[<ffff82d0801572f0>] cacheline_flush+0x4/0x9
    <snip>
    (XEN) Xen call trace:
    (XEN)    [<ffff82d0801572f0>] cacheline_flush+0x4/0x9
    (XEN)    [<ffff82d08014ffff>] __iommu_flush_cache+0x4a/0x6a
    (XEN)    [<ffff82d0801532e2>] iommu_pte_flush+0x2b/0xd5
    (XEN)    [<ffff82d0801f909a>] ept_set_entry+0x4bc/0x61f
    (XEN)    [<ffff82d0801f0c25>] p2m_set_entry+0xd1/0x112
    (XEN)    [<ffff82d0801f25b1>] clear_mmio_p2m_entry+0x1a0/0x200
    (XEN)    [<ffff82d0801f4aac>] unmap_mmio_regions+0x49/0x73
    (XEN)    [<ffff82d080106292>] do_domctl+0x15bd/0x1edb
    (XEN)    [<ffff82d080234fcb>] syscall_enter+0xeb/0x145
    (XEN)
    (XEN) Pagetable walk from ffff820040004ae0:
    (XEN)  L4[0x104] = 00000008668a5063 ffffffffffffffff
    (XEN)  L3[0x001] = 00000008668a3063 ffffffffffffffff
    (XEN)  L2[0x000] = 000000086689c063 ffffffffffffffff
    (XEN)  L1[0x004] = 000000056f078063 000000000007f678
    (XEN)
    (XEN) ****************************************
    (XEN) Panic on CPU 7:
    (XEN) FATAL PAGE FAULT
    (XEN) [error_code=0000]
    (XEN) Faulting linear address: ffff820040004ae0
    (XEN) ****************************************

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm/p2m-ept.c

@@ -801,8 +801,6 @@ ept_set_entry(struct p2m_domain *p2m, unsigned long gfn, mfn_t mfn,
         p2m->max_mapped_pfn = gfn + (1UL << order) - 1;
 
 out:
-    unmap_domain_page(table);
-
     if ( needs_sync != sync_off )
         ept_sync_domain(p2m);
 
@@ -825,6 +823,8 @@ out:
         }
     }
 
+    unmap_domain_page(table);
+
     /* Release the old intermediate tables, if any.  This has to be the
        last thing we do, after the ept_sync_domain() and removal
        from the iommu tables, so as to avoid a potential