IB/mlx5: Fix coverity warning

Fix covertiy warning of passing "&flow_attr" to function
"create_flow_rule" which uses it as an array.

In addition pass flow attributes argument as const.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>