iommufd: Rename some shortterm-related identifiers

iommufd: Rename some shortterm-related identifiers

Rename the shortterm-related identifiers to wait-related.

The usage of shortterm_users refcount is now beyond its name.  It is
also used for references which live longer than an ioctl execution.
E.g. vdev holds idev's shortterm_users refcount on vdev allocation,
releases it during idev's pre_destroy(). Rename the refcount as
wait_cnt, since it is always used to sync the referencing & the
destruction of the object by waiting for it to go to zero.

List all changed identifiers:

  iommufd_object::shortterm_users -> iommufd_object::wait_cnt
  REMOVE_WAIT_SHORTTERM -> REMOVE_WAIT
  iommufd_object_dec_wait_shortterm() -> iommufd_object_dec_wait()
  zerod_shortterm -> zerod_wait_cnt

No functional change intended.

Link: https://patch.msgid.link/r/20250716070349.1807226-9-yilun.xu@linux.intel.com
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>