iommufd: Make vfio_compat's unmap succeed if the range is already empty

iommufd: Make vfio_compat's unmap succeed if the range is already empty

iommufd returns ENOENT when attempting to unmap a range that is already
empty, while vfio type1 returns success. Fix vfio_compat to match.

Fixes: d624d6652a65 ("iommufd: vfio container FD ioctl compatibility")
Link: https://patch.msgid.link/r/0-v1-76be45eff0be+5d-iommufd_unmap_compat_jgg@nvidia.com
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Alex Mastro <amastro@fb.com>
Reported-by: Alex Mastro <amastro@fb.com>
Closes: https://lore.kernel.org/r/aP0S5ZF9l3sWkJ1G@devgpu012.nha5.facebook.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>