lib: kunit_iov_iter: improve error detection

lib: kunit_iov_iter: improve error detection

In the kunit_iov_iter test prevent the kernel buffer from being a single
physically contiguous region.

Additionally, make sure that the test pattern written to a page in the
buffer depends on the offset of the page within the buffer.

Link: https://lkml.kernel.org/r/20260326214905.818170-5-lk@c--e.de
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Cc: David Howells <dhowells@redhat.com>
Cc: David Gow <davidgow@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>