kunit: Move kunit_abort() call out of kunit_do_failed_assertion()

KUnit aborts the current thread when an assertion fails. Currently, this
is done conditionally as part of the kunit_do_failed_assertion()
function, but this hides the kunit_abort() call from the compiler
(particularly if it's in another module). This, in turn, can lead to
both suboptimal code generation (the compiler can't know if
kunit_do_failed_assertion() will return), and to static analysis tools
like smatch giving false positives.

Moving the kunit_abort() call into the macro should give the compiler
and tools a better chance at understanding what's going on. Doing so
requires exporting kunit_abort(), though it's recommended to continue to
use assertions in lieu of aborting directly.

In addition, kunit_abort() and kunit_do_failed_assertion() are renamed
to make it clear they they're intended for internal KUnit use, to:
__kunit_do_failed_assertion() and __kunit_abort()

Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>