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pc64: avoid getcr3() in mmuflushtlb()

it turns out that calculating physical address of pml4 is faster
than reading the machine register, so pass it explicitely.

--- a/sys/src/9/pc64/fns.h

+++ b/sys/src/9/pc64/fns.h

@@ -96,7 +96,7 @@

 void	meminit0(void);

 void	memreserve(uintptr, uintptr);

 void	mfence(void);

-#define mmuflushtlb() putcr3(getcr3())

+#define mmuflushtlb(pml4) putcr3(pml4)

 void	mmuinit(void);

 uintptr	*mmuwalk(uintptr*, uintptr, int, int);

 char*	mtrr(uvlong, uvlong, char *);

--- a/sys/src/9/pc64/main.c

+++ b/sys/src/9/pc64/main.c

@@ -239,7 +239,7 @@

 	if((pte = mmuwalk(m->pml4, REBOOTADDR, 0, 0)) != nil)

 		*pte &= ~PTENOEXEC;

-	mmuflushtlb();

+	mmuflushtlb(PADDR(m->pml4));

 	/* setup reboot trampoline function */

 	f = (void*)REBOOTADDR;

@@ -350,5 +350,5 @@

 	 * You might think it would be a win not to do this in that case,

 	 * especially on VMware, but it turns out not to matter.

 	 */

-	mmuflushtlb();

+	mmuflushtlb(PADDR(m->pml4));

 }

--- a/sys/src/9/pc64/mmu.c

+++ b/sys/src/9/pc64/mmu.c

@@ -67,7 +67,7 @@

 	tss->rsp1[1] = stack >> 32;

 	tss->rsp2[0] = (u32int)stack;

 	tss->rsp2[1] = stack >> 32;

-	mmuflushtlb();

+	mmuflushtlb(PADDR(m->pml4));

 }

 static void kernelro(void);