Fix regression in IA-64 caused by page table allocation refactoring

The refactoring and consolidation of <asm/pgalloc.h> caused regression
 on parisc and ia64. The fix for parisc made it into v5.9-rc1 while the
 fix ia64 got delayed a bit and here it is.
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Merge tag 'fixes-2020-08-18' of git://git.kernel.org/pub/scm/linux/kernel/git/rppt/memblock

Pull ia64 page table fix from Mike Rapoport:
 "Fix regression in IA-64 caused by page table allocation refactoring

  The refactoring and consolidation of <asm/pgalloc.h> caused regression
  on parisc and ia64. The fix for parisc made it into v5.9-rc1 while the
  fix ia64 got delayed a bit and here it is"

* tag 'fixes-2020-08-18' of git://git.kernel.org/pub/scm/linux/kernel/git/rppt/memblock:
  arch/ia64: Restore arch-specific pgd_offset_k implementation

arch/ia64/include/asm/pgtable.h

@@ -366,6 +366,15 @@ pgd_index (unsigned long address)
 }
 #define pgd_index pgd_index
 
+/*
+ * In the kernel's mapped region we know everything is in region number 5, so
+ * as an optimisation its PGD already points to the area for that region.
+ * However, this also means that we cannot use pgd_index() and we must
+ * never add the region here.
+ */
+#define pgd_offset_k(addr) \
+	(init_mm.pgd + (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1)))
+
 /* Look up a pgd entry in the gate area.  On IA-64, the gate-area
    resides in the kernel-mapped segment, hence we use pgd_offset_k()
    here.  */

include/linux/pgtable.h

@@ -117,7 +117,9 @@ static inline pgd_t *pgd_offset_pgd(pgd_t *pgd, unsigned long address)
  * a shortcut which implies the use of the kernel's pgd, instead
  * of a process's
  */
+#ifndef pgd_offset_k
 #define pgd_offset_k(address)		pgd_offset(&init_mm, (address))
+#endif
 
 /*
  * In many cases it is known that a virtual address is mapped at PMD or PTE