x86: convert BUG_ON()s to WARN_ON()s in read_descriptor()

In the light of AMD erratum #700, and given that these checks happen
for debugging purposes only and also only in debug builds of the
hypervisor, make the failures non-fatal.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1544,11 +1544,11 @@ static int read_descriptor(unsigned int 
             asm volatile (
                 "larl %2,%0 ; setz %1"
                 : "=r" (a), "=qm" (valid) : "rm" (sel));
-            BUG_ON(valid && ((a & 0x00f0ff00) != *ar));
+            WARN_ON(valid && ((a & 0x00f0ff00) != *ar));
             asm volatile (
                 "lsll %2,%0 ; setz %1"
                 : "=r" (l), "=qm" (valid) : "rm" (sel));
-            BUG_ON(valid && (l != *limit));
+            WARN_ON(valid && (l != *limit));
         }
 #endif
     }