net/mlx5: limit the function in local scope

The function mlx5_buf_alloc_node is only used by the function in the
local scope. So it is appropriate to limit this function in the local
scope.

Signed-off-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>