test_printf: test printf family at runtime

This adds a simple module for testing the kernel's printf facilities.
Previously, some %p extensions have caused a wrong return value in case
the entire output didn't fit and/or been unusable in kasprintf().  This
should help catch such issues.  Also, it should help ensure that changes
to the formatting algorithms don't break anything.

I'm not sure if we have a struct dentry or struct file lying around at
boot time or if we can fake one, but most %p extensions should be
testable, as should the ordinary number and string formatting.

The nature of vararg functions means we can't use a more conventional
table-driven approach.

For now, this is mostly a skeleton; contributions are very
welcome. Some tests are/will be slightly annoying to write, since the
expected output depends on stuff like CONFIG_*, sizeof(long), runtime
values etc.

Signed-off-by: Rasmus Villemoes <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Martin Kletzander <[email protected]>
Cc: Rasmus Villemoes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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