Before modifying how the __packed attribute is used, add compile time
size checks for the structures that will be modified.

Link: https://lore.kernel.org/r/20210524041211.9480-3-bvanassche@acm.orgReviewed-by: Leon Romanovsky <leonro@nvidia.com>
Cc: Nicolas Morey-Chaisemartin <nmoreychaisemartin@suse.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>