Convert tests of panicstr to KERNEL_PANICKED().

[tmunro]

Summary

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.

[tmunro]

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).

[allanjude]

Future note: please upload patches with full context, either using the 'arcanist' tool, or just generate the diff with -U99999 (svn diff -x -U99999)

[allanjude]

Approved By: allanjude

[mjg]

with this patch applied git grep panicstr still reveals entries in cddl/ code. it should also get modified or the boolean switch is not going to work.

[tmunro]

Discussed with Mateusz, he would prefer to write a Coccinelle script to automate this change, so I am abandoning this patch.