iommufd/selftest: Update the fail_nth limit

There are more failure conditions now so 400 iterations is not enough pass
them all, up it to 1000. The limit exists so it doesn't infinite loop.

Link: https://patch.msgid.link/r/3-v1-02cd136829df+31-iommufd_syz_fput_jgg@nvidia.com


Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>