iommufd/selftest: Check for overflow in IOMMU_TEST_OP_ADD_RESERVED

In the Linux kernel, the following vulnerability has been resolved:

iommufd/selftest: Check for overflow in IOMMU_TEST_OP_ADD_RESERVED

syzkaller found it could overflow math in the test infrastructure and cause a WARN_ON by corrupting the reserved interval tree. This only effects test kernels with CONFIG_IOMMUFD_TEST.

Validate the user input length in the test ioctl.