x86/mm: build map_domain_gfn() just once

It doesn't depend on GUEST_PAGING_LEVELS.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
TBD: To ensure no dependency on GUEST_PAGING_LEVELS, would it perhaps
     be better to move the function to a file compiled just once?

--- a/xen/arch/x86/mm/guest_walk.c
+++ b/xen/arch/x86/mm/guest_walk.c
@@ -87,8 +87,11 @@ static uint32_t set_ad_bits(void *guest_
     return 0;
 }
 
-/* If the map is non-NULL, we leave this function having 
- * acquired an extra ref on mfn_to_page(*mfn) */
+#if GUEST_PAGING_LEVELS == CONFIG_PAGING_LEVELS
+/*
+ * If the map is non-NULL, we leave this function having
+ * acquired an extra ref on mfn_to_page(*mfn).
+ */
 void *map_domain_gfn(struct p2m_domain *p2m, gfn_t gfn, mfn_t *mfn,
                      p2m_type_t *p2mt, p2m_query_t q, uint32_t *rc)
 {
@@ -125,7 +128,7 @@ void *map_domain_gfn(struct p2m_domain *
     map = map_domain_page(*mfn);
     return map;
 }
-
+#endif
 
 /* Walk the guest pagetables, after the manner of a hardware walker. */
 /* Because the walk is essentially random, it can cause a deadlock 
--- a/xen/include/asm-x86/guest_pt.h
+++ b/xen/include/asm-x86/guest_pt.h
@@ -305,7 +305,6 @@ guest_walk_to_page_order(walk_t *gw)
 #define GPT_RENAME2(_n, _l) _n ## _ ## _l ## _levels
 #define GPT_RENAME(_n, _l) GPT_RENAME2(_n, _l)
 #define guest_walk_tables GPT_RENAME(guest_walk_tables, GUEST_PAGING_LEVELS)
-#define map_domain_gfn GPT_RENAME(map_domain_gfn, GUEST_PAGING_LEVELS)
 
 void *map_domain_gfn(struct p2m_domain *p2m, gfn_t gfn, mfn_t *mfn,
                      p2m_type_t *p2mt, p2m_query_t q, uint32_t *rc);