KVM: x86/mmu: Flush remote TLBs iff MMU-writable flag is cleared from RO SPTE

KVM: x86/mmu: Flush remote TLBs iff MMU-writable flag is cleared from RO SPTE

Don't force a remote TLB flush if KVM happens to effectively "refresh" a
read-only SPTE that is still MMU-Writable, as KVM allows MMU-Writable SPTEs
to have Writable TLB entries, even if the SPTE is !Writable.  Remote TLBs
need to be flushed only when creating a read-only SPTE for write-tracking,
i.e. when installing a !MMU-Writable SPTE.

In practice, especially now that KVM doesn't overwrite existing SPTEs when
prefetching, KVM will rarely "refresh" a read-only, MMU-Writable SPTE,
i.e. this is unlikely to eliminate many, if any, TLB flushes.  But, more
precisely flushing makes it easier to understand exactly when KVM does and
doesn't need to flush.

Note, x86 architecturally requires relevant TLB entries to be invalidated
on a page fault, i.e. there is no risk of putting a vCPU into an infinite
loop of read-only page faults.

Cc: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20241011021051.1557902-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>