Merge branch 'vsock-virtio-fix-msg_peek-calculation-on-bytes-to-copy'

Luigi Leonardi says:

====================
vsock/virtio: fix MSG_PEEK calculation on bytes to copy

`virtio_transport_stream_do_peek`, when calculating the number of bytes to
copy, didn't consider the `offset`, caused by partial reads that happened
before.
This might cause out-of-bounds read that lead to an EFAULT.
More details in the commits.

Commit 1 introduces the fix
Commit 2 introduces some preliminary work for adding a test and fixes a
problem in existing tests.
Commit 3 introduces a test that checks for this bug to avoid future
regressions.

For disclosure: this bug was found initially by claude opus 4.6, I then analyzed
it and worked on the fix and the test.
====================

Link: https://patch.msgid.link/20260415-fix_peek-v4-0-8207e872759e@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>