The current kcov code attaches a kcov object to the thread which opened
it. This is not desirable, as syzkaller opens /dev/kcov in a parent
process and passes it down to the child.
Move the thread attachment from open() time to start time.
Implement a device (/dev/kcov) that exposes coverage info for the
kernel. This will be especially useful for coverage-based fuzzing (e.g.
syzkaller).
Exclude a lot of functions which would crash/otherwise interfere in the
coverage, including only processing coverage callbacks when not handling
an irq.