dtrace/amd64: Make invop providers usable with KMSAN enabled

- Use a fresh context when entering dtrace_invop() via a breakpoint
  exception.
- Mark the #BP trapframe as initialized.

MFC after:	2 weeks

sys/cddl/dev/dtrace/amd64/dtrace_asm.S

@@ -58,6 +58,8 @@
 
 	ENTRY(dtrace_invop_start)
 
+	KMSAN_ENTER
+
 	/*
 	 * #BP traps with %rip set to the next address. We need to decrement
 	 * the value to indicate the address of the int3 (0xcc) instruction
@@ -77,6 +79,12 @@
 	call	dtrace_invop
 	addq	$16, %rsp
 
+#ifdef KMSAN
+	movq	%rax, %r12
+	KMSAN_LEAVE
+	movq	%r12, %rax
+#endif
+
 	cmpl	$DTRACE_INVOP_PUSHL_EBP, %eax
 	je	bp_push
 	cmpl	$DTRACE_INVOP_CALL, %eax

sys/cddl/dev/dtrace/amd64/dtrace_subr.c

@@ -33,6 +33,7 @@
 #include <sys/systm.h>
 #include <sys/kernel.h>
 #include <sys/malloc.h>
+#include <sys/msan.h>
 #include <sys/proc.h>
 #include <sys/smp.h>
 #include <sys/dtrace_impl.h>
@@ -68,6 +69,8 @@ dtrace_invop(uintptr_t addr, struct trapframe *frame, void **scratch)
 	dtrace_invop_hdlr_t *hdlr;
 	int rval;
 
+	kmsan_mark(frame, sizeof(*frame), KMSAN_STATE_INITED);
+
 	td = curthread;
 	td->t_dtrace_trapframe = frame;
 	rval = 0;