Many code paths check if the kernel has panicked by accessing
panicstr. Change them all to use a new function-like macro
KERNEL_PANICKED(). This lets us use __predict_false()
consistently, and lets us make further changes to the way the
condition is detected in future.
Here's a better version (the last one had an unintended stray change).
Note: this is a refactoring suggested by mjg, to support his future plans to change the representation of panic state to something small in __read_frequently. The new macro will hide those details from all these call sites, though for now it just does what it was doing before (with the addition of the prediction).