iommufd: Add some fault injection points

This increases the coverage the fail_nth test gets, as well as via
syzkaller.

Link: https://lore.kernel.org/r/17-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com


Tested-by: Matthew Rosato <mjrosato@linux.ibm.com> # s390
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>