tile: avoid recursive backtrace faults

This change adds support for avoiding recursive backtracer crashes;
we haven't seen this in practice other than when things are seriously
corrupt, but it may help avoid losing the root cause of a crash.

Also, don't abort kernel backtracers for invalid userspace PC's.
If we do, we lose the ability to backtrace through a userspace
call to a bad address above PAGE_OFFSET, even though that it can
be perfectly reasonable to continue the backtrace in such a case.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/include/asm/processor.h

@@ -110,6 +110,8 @@ struct thread_struct {
 	unsigned long long interrupt_mask;
 	/* User interrupt-control 0 state */
 	unsigned long intctrl_0;
+	/* Is this task currently doing a backtrace? */
+	bool in_backtrace;
 #if CHIP_HAS_PROC_STATUS_SPR()
 	/* Any other miscellaneous processor state bits */
 	unsigned long proc_status;