x86: fix paging_max_paddr_bits()

paging_max_paddr_bits() has an invalid use of IS_ENABLED(): instead of
IS_ENABLED(CONFIG_BIGMEM) it is using IS_ENABLED(BIGMEM). Fix that.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/asm-x86/paging.h

@@ -371,7 +371,8 @@ static inline unsigned int paging_max_paddr_bits(const struct domain *d)
 {
     unsigned int bits = paging_mode_hap(d) ? hap_paddr_bits : paddr_bits;
 
-    if ( !IS_ENABLED(BIGMEM) && paging_mode_shadow(d) && !is_pv_domain(d) )
+    if ( !IS_ENABLED(CONFIG_BIGMEM) && paging_mode_shadow(d) &&
+         !is_pv_domain(d) )
     {
         /* Shadowed superpages store GFNs in 32-bit page_info fields. */
         bits = min(bits, 32U + PAGE_SHIFT);