net/mlx5: DR, prevent potential error pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5: DR, prevent potential error pointer dereference

The dr_domain_add_vport_cap() function generally returns NULL on error

but sometimes we want it to return ERR_PTR(-EBUSY) so the caller can

retry. The problem here is that "ret" can be either -EBUSY or -ENOMEM

and if it's and -ENOMEM then the error pointer is propogated back and

eventually dereferenced in dr_ste_v0_build_src_gvmi_qpn_tag().