Remove custom and duplicated variant of offsetofend().

Link: https://lore.kernel.org/r/20200310091438.248429-4-leon@kernel.orgSigned-off-by: NLeon Romanovsky <leonro@mellanox.com>
Acked-by: NGal Pressman <galpress@amazon.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>