KVM: x86/xen: Fix eventfd error handling in kvm_xen_eventfd_assign()

Should not call eventfd_ctx_put() in case of error.

Fixes: 2fd6df2f ("KVM: x86/xen: intercept EVTCHNOP_send from guests")
Reported-by:  <syzbot+6f0c896c5a9449a10ded@syzkaller.appspotmail.com>
Signed-off-by: Eiichi Tsukata <eiichi.tsukata@nutanix.com>
Message-Id: <20221028092631.117438-1-eiichi.tsukata@nutanix.com>
[Introduce new goto target instead. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>