RDMA/rxe: Remove redundant err variable

Return value directly instead of taking this in another redundant
variable.

Link: https://lore.kernel.org/20211215075258.442930-1-chi.minghao@zte.com.cn
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Acked-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Reviewed-by: Devesh Sharma <Devesh.s.sharma@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>