KVM: arm64: Only drop references on empty tables in stage2_free_walker

KVM: arm64: Only drop references on empty tables in stage2_free_walker

A subsequent change to the way KVM frees stage-2s will invoke the free
walker on sub-ranges of the VM's IPA space, meaning there's potential
for only partially visiting a table's PTEs.

Split the leaf and table visitors and only drop references on a table
when the page count reaches 1, implying there are no valid PTEs that
need to be visited. Invalidate the table PTE to avoid traversing the
stale reference.

Link: https://msgid.link/20251113052452.975081-2-rananta@google.com
Signed-off-by: Oliver Upton <oupton@kernel.org>