x86_64: Correct phys_addr in cleanup_highmap comment

For x86_64, we have phys_base, which means the delta between the
the address kernel is actually running at and the address kernel
is compiled to run at. Not phys_addr so correct it.
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/5192F9BF.2000802@cn.fujitsu.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/init_64.c

@@ -368,7 +368,7 @@ void __init init_extra_mapping_uc(unsigned long phys, unsigned long size)
  *
  *   from __START_KERNEL_map to __START_KERNEL_map + size (== _end-_text)
  *
- * phys_addr holds the negative offset to the kernel, which is added
+ * phys_base holds the negative offset to the kernel, which is added
  * to the compile time generated pmds. This results in invalid pmds up
  * to the point where we hit the physaddr 0 mapping.
  *