KVM: x86: do not report preemption if the steal time cache is stale

Commit 7e2175eb ("KVM: x86: Fix recording of guest steal time
/ preempted status", 2021-11-11) open coded the previous call to
kvm_map_gfn, but in doing so it dropped the comparison between the cached
guest physical address and the one in the MSR.  This cause an incorrect
cache hit if the guest modifies the steal time address while the memslots
remain the same.  This can happen with kexec, in which case the preempted
bit is written at the address used by the old kernel instead of
the old one.

Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: stable@vger.kernel.org
Fixes: 7e2175eb

 ("KVM: x86: Fix recording of guest steal time / preempted status")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>