arch/ia64: Restore arch-specific pgd_offset_k implementation

IA-64 is special and treats pgd_offset_k() differently to pgd_offset(),
using different formulae to calculate the indices into the kernel and user
PGDs.  The index into the user PGDs takes into account the region number,
but the index into the kernel (init_mm) PGD always assumes a predefined
kernel region number. Commit 974b9b2c ("mm: consolidate pte_index() and
pte_offset_*() definitions") made IA-64 use a generic pgd_offset_k() which
incorrectly used pgd_index() for kernel page tables.  As a result, the
index into the kernel PGD was going out of bounds and the kernel hung
during early boot.

Allow overrides of pgd_offset_k() and override it on IA-64 with the old
implementation that will correctly index the kernel PGD.

Fixes: 974b9b2c ("mm: consolidate pte_index() and pte_offset_*() definitions")
Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Acked-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>

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