RDMA/ucma: Correct option size check using optlen

The option size check is using optval instead of optlen
causing the set option call to fail. Use the correct
field, optlen, for size check.

Fixes: 6a21dfc0 ("RDMA/ucma: Limit possible option size")
Signed-off-by: Chien Tin Tung <chien.tin.tung@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>