xvmalloc: adjust XVFREE() ordering

What c4f427ec87 ("xen: Swap order of actions in the FREE*() macros") did
should have been done right away when XVFREE() was introduced.

Fixes: 9102fcd957 ("mm: introduce xvmalloc() et al and use for grant table allocations")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

xen/include/xen/xvmalloc.h

@@ -71,8 +71,9 @@ void *_xvrealloc(void *va, size_t size, unsigned int align);
 
 /* Free an allocation, and zero the pointer to it. */
 #define XVFREE(p) do { \
-    xvfree(p);         \
+    void *_ptr_ = (p); \
     (p) = NULL;        \
+    xvfree(_ptr_);     \
 } while ( false )
 
 static inline void *_xvmalloc_array(