net/mlx5: handle errors in mlx5_chains_create_table()

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

net/mlx5: handle errors in mlx5_chains_create_table()

In mlx5_chains_create_table(), the return value of mlx5_get_fdb_sub_ns()

and mlx5_get_flow_namespace() must be checked to prevent NULL pointer

dereferences. If either function fails, the function should log error

message with mlx5_core_warn() and return error pointer.