iommufd/selftest: Fix page-size check in iommufd_test_dirty()

iommufd_test_dirty()/IOMMU_TEST_OP_DIRTY sets the dirty bits in the mock
domain implementation that the userspace side validates against what it
obtains via the UAPI.

However in introducing iommufd_test_dirty() it forgot to validate page_size
being 0 leading to two possible divide-by-zero problems: one at the
beginning when calculating @max and while calculating the IOVA in the
XArray PFN tracking list.

While at it, validate the length to require non-zero value as well, as we
can't be allocating a 0-sized bitmap.

Link: https://lore.kernel.org/r/20231030113446.7056-1-joao.m.martins@oracle.com
Reported-by:  <syzbot+25dc7383c30ecdc83c38@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/linux-iommu/00000000000005f6aa0608b9220f@google.com/
Fixes: a9af47e3

 ("iommufd/selftest: Test IOMMU_HWPT_GET_DIRTY_BITMAP")
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>