KVM: Drop bogus "pfn != 0" guard from kvm_release_pfn()

Remove a check from kvm_release_pfn() to bail if the provided @pfn is
zero.  Zero is a perfectly valid pfn on most architectures, and should
not be used to indicate an error or an invalid pfn.  The bogus check was
added by commit 91724814 ("x86/kvm: Cache gfn to pfn translation"),
which also did the bad thing of zeroing the pfn and gfn to mark a cache
invalid.  Thankfully, that bad behavior was axed by commit 357a18ad
("KVM: Kill kvm_map_gfn() / kvm_unmap_gfn() and gfn_to_pfn_cache").

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>