KVM: Harden and prepare for modifying existing guest_memfd memslots

KVM: Harden and prepare for modifying existing guest_memfd memslots

Unbind guest_memfd memslots if KVM commits a MOVE or FLAGS_ONLY memslot
change to harden against use-after-free, and to prepare for eventually
supporting dirty logging on guest_memfd memslots, at which point
FLAGS_ONLY changes will be expected/supported.

Add two separate WARNs, once to yell if a guest_memfd memslot is moved
(which KVM is never expected to allow/support), and again if the unbind()
is triggered, to help detect uAPI goofs prior to deliberately allowing
FLAGS_ONLY changes.

Link: https://patch.msgid.link/20251202020334.1171351-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>