KVM: Write the per-page "segment" when clearing (part of) a guest page

Pass "seg" instead of "len" when writing guest memory in kvm_clear_guest(),
as "seg" holds the number of bytes to write for the current page, while
"len" holds the total bytes remaining.

Luckily, all users of kvm_clear_guest() are guaranteed to not cross a page
boundary, and so the bug is unhittable in the current code base.

Fixes: 2f541442 ("KVM: remove kvm_clear_guest_page")
Reported-by:  <zyr_ms@outlook.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219104
Link: https://lore.kernel.org/r/20240829191413.900740-2-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>