x86/debug: Change thread.debugreg6 to thread.virtual_dr6

Current usage of thread.debugreg6 is convoluted at best. It starts life as
a copy of the hardware DR6 value, but then various bits are cleared and
set.

Replace this with a new variable thread.virtual_dr6 that is initialized to
0 when DR6 is read and only gains bits, at the same time the actual (on
stack) dr6 value which is read from the hardware only gets bits cleared.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.415372940@infradead.org

arch/x86/include/asm/processor.h

@@ -517,7 +517,7 @@ struct thread_struct {
 	/* Save middle states of ptrace breakpoints */
 	struct perf_event	*ptrace_bps[HBP_NUM];
 	/* Debug status used for traps, single steps, etc... */
-	unsigned long           debugreg6;
+	unsigned long           virtual_dr6;
 	/* Keep track of the exact dr7 value set by the user */
 	unsigned long           ptrace_dr7;
 	/* Fault info: */