powerpc/hash64: Restrict page table lookup using init_mm with __flush_hash_table_range

This is only used with init_mm currently. Walking init_mm is much simpler
because we don't need to handle concurrent page table like other mm_context

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200505071729.54912-5-aneesh.kumar@linux.ibm.com

arch/powerpc/kernel/pci_64.c

@@ -100,7 +100,7 @@ int pcibios_unmap_io_space(struct pci_bus *bus)
 			 pci_name(bus->self));
 
 #ifdef CONFIG_PPC_BOOK3S_64
-		__flush_hash_table_range(&init_mm, res->start + _IO_BASE,
+		__flush_hash_table_range(res->start + _IO_BASE,
 					 res->end + _IO_BASE + 1);
 #endif
 		return 0;