KVM: SEV: Mark source page dirty when writing back CPUID data on failure

KVM: SEV: Mark source page dirty when writing back CPUID data on failure

When writing back CPUID data (provided by trusted firmware) to the source
page on failure, mark the page/folio as dirty so that the data isn't lost
in the unlikely scenario the page is reclaimed before its read by
userspace.

Fixes: 2a62345b3052 ("KVM: guest_memfd: GUP source pages prior to populating guest memory")
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260522-fix-sev-gmem-post-populate-v2-5-3f196bfad5a1@google.com
[sean: use set_page_dirty(), massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>