iommufd/selftest: Fix buffer read overrrun in the dirty test

iommufd/selftest: Fix buffer read overrrun in the dirty test

test_bit() is used to read the memory storing the bitmap, however
test_bit() always uses a unsigned long 8 byte access.

If the bitmap is not an aligned size of 64 bits this will now trigger a
KASAN warning reading past the end of the buffer.

Properly round the buffer allocation to an unsigned long size. Continue to
copy_from_user() using a byte granularity.

Fixes: 9560393b830b ("iommufd/selftest: Fix iommufd_test_dirty() to handle <u8 bitmaps")
Link: https://patch.msgid.link/r/0-v1-113e8d9e7861+5ae-iommufd_kasan_jgg@nvidia.com
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>