KVM: x86/mmu: Rename __gfn_to_rmap to gfn_to_rmap

gfn_to_rmap was removed in the previous patch so there is no need to
retain the double underscore on __gfn_to_rmap.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210804222844.1419481-7-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>