lib: kunit_iov_iter: add tests for extract_iter_to_sg

lib: kunit_iov_iter: add tests for extract_iter_to_sg

Add test cases that test extract_iter_to_sg.

For each iterator type an iterator is loaded with a suitable buffer.  The
iterator is then extracted to a scatterlist with multiple calls to
extract_iter_to_sg.  The final scatterlist is copied into a scratch
buffer.

The test passes if the scratch buffer contains the same data as the
original buffer.

The new tests demonstrate bugs in extract_iter_to_sg for kvec and user
iterators that are fixed by the previous commits.

Link: https://lkml.kernel.org/r/20260326214905.818170-6-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>