iommufd: Add a pre_destroy() op for objects

Add a pre_destroy() op which gives objects a chance to clear their
short term users references before destruction. This op is intended for
external driver created objects (e.g. idev) which does deterministic
destruction.

In order to manage the lifecycle of interrelated objects as well as the
deterministic destruction (e.g. vdev can't outlive idev, and idev
destruction can't fail), short term users references are allowed to
live out of an ioctl execution. An immediate use case is, vdev holds
idev's short term user reference until vdev destruction completes, idev
leverages existing wait_shortterm mechanism to ensure it is destroyed
after vdev.

This extended usage makes the referenced object unable to just wait for
its reference gone. It needs to actively trigger the reference removal,
as well as prevent new references before wait. Should implement these
work in pre_destroy().

Link: https://patch.msgid.link/r/[email protected]
Suggested-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Nicolin Chen <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Tested-by: Nicolin Chen <[email protected]>
Signed-off-by: Xu Yilun <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

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