iommufd/selftest: Check for overflow in IOMMU_TEST_OP_ADD_RESERVED

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.

Fixes: f4b20bb34c83 ("iommufd: Add kernel support for testing iommufd")
Link: https://patch.msgid.link/r/0-v1-cd99f6049ba5+51-iommufd_syz_add_resv_jgg@nvidia.com
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Yi Liu <yi.l.liu@intel.com>
Reported-by: syzbot+57fdb0cf6a0c5d1f15a2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69368129.a70a0220.38f243.008f.GAE@google.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>