KVM: Put the extra pfn reference when reusing a pfn in the gpc cache

Put the struct page reference to pfn acquired by hva_to_pfn() when the
old and new pfns for a gfn=>pfn cache match.  The cache already has a
reference via the old/current pfn, and will only put one reference when
the cache is done with the pfn.

Fixes: 982ed0de

 ("KVM: Reinstate gfn_to_pfn_cache with invalidation support")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429210025.3293691-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>