iommufd: Fix error pointer checking

Smatch static checker reported below warning:

    drivers/iommu/iommufd/fault.c:131 iommufd_device_get_attach_handle()
    warn: 'handle' is an error pointer or valid

Fix it by checking 'handle' with IS_ERR().

Fixes: b7d88336 ("iommufd: Fault-capable hwpt attach/detach/replace")
Link: https://lore.kernel.org/r/20240712025819.63147-1-baolu.lu@linux.intel.com


Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-iommu/8bb4f37a-4514-4dea-aabb-7380be303895@stanley.mountain/


Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>