x86/mem-sharing: ensure consistent lock order in get_two_gfns()

While the comment validly says "Sort by domain, if same domain by gfn",
the implementation also included equal domain IDs in the first part of
the check, thus rending the second part entirely dead and leaving
deadlock potential when there's only a single domain involved.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
master commit: 09af2d01a2
master date: 2021-07-07 12:35:12 +0200

xen/include/asm-x86/p2m.h

@@ -576,7 +576,7 @@ do {                                                    \
     dest ## _t   = (source ## t)   ?: &scratch_t;       \
 } while (0)
 
-    if ( (rd->domain_id <= ld->domain_id) ||
+    if ( (rd->domain_id < ld->domain_id) ||
          ((rd == ld) && (gfn_x(rgfn) <= gfn_x(lgfn))) )
     {
         assign_pointers(first, r);