net/mlx5: handle errors in mlx5_chains_create_table()

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.

Fixes: 39ac237ce009 ("net/mlx5: E-Switch, Refactor chains and priorities")
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250307021820.2646-1-vulab@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>