RDMA/rw: use DIV_ROUND_UP to calculate nr_ops

Don't open-code DIV_ROUND_UP() kernel macro.

Link: https://lore.kernel.org/r/20200413133905.933343-1-leon@kernel.org


Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>