[BUG] vdpa: use-after-free and WARN in _vdpa_unregister_device()

Hello,

As part of the kernel research at ASU's SEFCOM
lab, we hit the crash below. Crash reports can be found here:

  https://github.com/farhad-alemi/public_bug_reports/tree/main/163-warning-in-vdpa-unregister-device/

  BUG: KASAN: slab-use-after-free in _vdpa_unregister_device+0x35/0x60
drivers/vdpa/vdpa.c:250
  Read of size 8 at addr ffff88810eeef350 by task syz.1.2/11300
  Call Trace:
   _vdpa_unregister_device+0x35/0x60 drivers/vdpa/vdpa.c:250
   vp_vdpa_dev_del+0x3d/0x60 drivers/vdpa/virtio_pci/vp_vdpa.c:593
   vdpa_match_remove+0x97/0xb0 drivers/vdpa/vdpa.c:321
   bus_for_each_dev+0x23a/0x2c0 drivers/base/bus.c:383
   vdpa_mgmtdev_unregister+0x110/0x130 drivers/vdpa/vdpa.c:332
   vp_vdpa_remove+0x5e/0xa0 drivers/vdpa/virtio_pci/vp_vdpa.c:683
   pci_device_remove+0xa3/0x250 drivers/pci/pci-driver.c:512
   device_release_driver_internal+0x464/0x860 drivers/base/dd.c:1375

  WARNING: drivers/vdpa/vdpa.c:250 at
_vdpa_unregister_device+0x51/0x60 drivers/vdpa/vdpa.c:250, CPU#0:
syz.8.826/22808

Our reproducer.c is available upon request.

Happy to test a patch if that would help.

Regards,