iommufd/selftest: Do not leak the hwpt if IOMMU_TEST_OP_MD_CHECK_MAP fails

iommufd/selftest: Do not leak the hwpt if IOMMU_TEST_OP_MD_CHECK_MAP fails

If the input validation fails it returned without freeing the hwpt
refcount causing a leak. This triggers a WARN_ON when closing the fd:

  WARNING: drivers/iommu/iommufd/main.c:369 at iommufd_fops_release+0x385/0x430, CPU#1: repro/724

Found by szykaller.

Fixes: e93d5945ed5b ("iommufd: Change the selftest to use iommupt instead of xarray")
Link: https://patch.msgid.link/r/0-v1-c8ed57e24380+44ae-iommufd_selftest_hwpt_leak_jgg@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Closes: https://lore.kernel.org/r/aTJGMaqwQK0ASj0G@ly-workstation
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>