IB/mlx5: Fix transport-domain rollback and initialize lb mutex earlier

In the Linux kernel, the following vulnerability has been resolved: IB/mlx5: Fix transport-domain rollback and initialize lb mutex earlier mlx5_ib_alloc_transport_domain() allocates a transport domain and then may fail in mlx5_ib_enable_lb(). In that case, the allocated TD is leaked. Fix this by deallocating the TD when mlx5_ib_enable_lb() returns an error. Also return 0 explicitly in the no-loopback-capability success branch, and move dev->lb.mutex initialization to mlx5_ib_stage_init_init().