CVE-2026-74397: 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

mlx5iballoctransportdomain() allocates a transport domain and then may fail in mlx5ibenablelb(). In that case, the allocated TD is leaked.

Fix this by deallocating the TD when mlx5ibenablelb() returns an error. Also return 0 explicitly in the no-loopback-capability success branch, and move dev->lb.mutex initialization to mlx5ibstageinitinit().