IB/mlx4: Use list_for_each_entry_safe

Simplify the code in search_relocate_mgid0_group with by using
list_for_each_entry_safe().

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>