RDMA/core: Make FD destroy callback void

All FD object destroy implementations return 0, so declare this callback
void.

Link: https://lore.kernel.org/r/20201104144556.3809085-3-leon@kernel.org


Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>