x86/fixmap: Cleanup outdated comments

Remove stale comments and fix the not longer valid pagetable entry
reference.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190809114612.2569-1-caoj.fnst@cn.fujitsu.com