iommufd/selftest: Test u64 unaligned bitmaps

Exercise the dirty tracking bitmaps with byte unaligned addresses in
addition to the PAGE_SIZE unaligned bitmaps, using a address towards the
end of the page boundary.

In doing so, increase the tailroom we allocate for the bitmap from
MOCK_PAGE_SIZE(2K) into PAGE_SIZE(4K), such that we can test end of bitmap
boundary.

Link: https://lore.kernel.org/r/20240202133415.23819-4-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>