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.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>