x86/gnttab: fix asm() operand in gnttab_clear_flag()

The operand needs to use the 'w' modifier in case the compiler happens
to pick a register (which apparently it does for no-one but the
reporter of this problem).

Reported-by: Lin Ming <mlin@xxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -54,7 +54,7 @@ static inline void gnttab_clear_flag(uns
      * Note that this cannot be clear_bit(), as the access must be
      * confined to the specified 2 bytes.
      */
-    asm volatile ("lock btrw %1,%0" : "=m" (*st) : "Ir" (nr), "m" (*st));
+    asm volatile ("lock btrw %w1,%0" : "=m" (*st) : "Ir" (nr), "m" (*st));
 }
 
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