lib/scatterlist: fix length calculations in extract_kvec_to_sg

In the Linux kernel, the following vulnerability has been resolved:

lib/scatterlist: fix length calculations in extract_kvec_to_sg

Patch series "Fix bugs in extract_iter_to_sg()", v3.

Fix bugs in the kvec and user variants of extract_iter_to_sg. This series

is growing due to useful remarks made by sashiko.dev.

The main bugs are:

- The length for an sglist entry when extracting from

a kvec can exceed the number of bytes in the page. This

is obviously not intended.

- When extracting a user buffer the sglist is temporarily

used as a scratch buffer for extracted page pointers.

If the sglist already contains some elements this scratch

buffer could overlap with existing entries in the sglist.

The series adds test cases to the kunit_iov_iter test that demonstrate all

of these bugs. Additionally, there is a memory leak fix for the test

itself.

The bugs were orignally introduced into kernel v6.3 where the function

lived in fs/netfs/iterator.c. It was later moved to lib/scatterlist.c in

v6.5. Thus the actual fix is only marked for backports to v6.5+.

This patch (of 5):

When extracting from a kvec to a scatterlist, do not cross page

boundaries. The required length was already calculated but not used as

intended.

Adjust the copied length if the loop runs out of sglist entries without

extracting everything.

While there, return immediately from extract_iter_to_sg if there are no

sglist entries at all.

A subsequent commit will add kunit test cases that demonstrate that the

patch is necessary.