RDMA/ipoib: Use min_t() macro instead of min()

Use min_t() macro to avoid the casting when using min() macro, also fix
the type of "length" and "wc->byte_len" to be "unsigned int" and
"u32" which is the right type for each one of them.

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>