x86/mm: do not attempt to convert _PAGE_GNTTAB to a boolean

Clang 10 complains with:

mm.c:1239:10: error: converting the result of '<<' to a boolean always evaluates to true
      [-Werror,-Wtautological-constant-compare]
    if ( _PAGE_GNTTAB && (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
         ^
xen/include/asm/x86_64/page.h:161:25: note: expanded from macro '_PAGE_GNTTAB'
#define _PAGE_GNTTAB (1U<<22)
                        ^

Remove the conversion of _PAGE_GNTTAB to a boolean and instead use a
preprocessor conditional to check if _PAGE_GNTTAB is defined.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 6eb61b1a9d
master date: 2020-06-02 13:36:41 +0200

xen/arch/x86/mm.c

@@ -1236,8 +1236,14 @@ void put_page_from_l1e(l1_pgentry_t l1e, struct domain *l1e_owner)
      *
      * (Note that the undestroyable active grants are not a security hole in
      * Xen. All active grants can safely be cleaned up when the domain dies.)
+     *
+     * NB: the preprocessor conditional is required in order to prevent clang's
+     * -Wtautological-constant-compare complaining about converting the result
+     * of a << into a bool is always true if it's evaluated directly in the if
+     * condition.
      */
-    if ( _PAGE_GNTTAB && (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
+#if _PAGE_GNTTAB
+    if ( (l1e_get_flags(l1e) & _PAGE_GNTTAB) &&
          !l1e_owner->is_shutting_down && !l1e_owner->is_dying )
     {
         gdprintk(XENLOG_WARNING,
@@ -1245,6 +1251,7 @@ void put_page_from_l1e(l1_pgentry_t l1e, struct domain *l1e_owner)
                  l1e_get_intpte(l1e));
         domain_crash(l1e_owner);
     }
+#endif
 
     /*
      * Remember we didn't take a type-count of foreign writable mappings