KVM: eventfd: fix NULL deref irqbypass consumer

Reported syzkaller:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
    IP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    PGD 0

    Oops: 0002 [#1] SMP
    CPU: 1 PID: 125 Comm: kworker/1:1 Not tainted 4.9.0+ #1
    Workqueue: kvm-irqfd-cleanup irqfd_shutdown [kvm]
    task: ffff9bbe0dfbb900 task.stack: ffffb61802014000
    RIP: 0010:irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    Call Trace:
     irqfd_shutdown+0x66/0xa0 [kvm]
     process_one_work+0x16b/0x480
     worker_thread+0x4b/0x500
     kthread+0x101/0x140
     ? process_one_work+0x480/0x480
     ? kthread_create_on_node+0x60/0x60
     ret_from_fork+0x25/0x30
    RIP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass] RSP: ffffb61802017e20
    CR2: 0000000000000008

The syzkaller folks reported a NULL pointer dereference that due to
unregister an consumer which fails registration before. The syzkaller
creates two VMs w/ an equal eventfd occasi...