IB/mlx4: Take sizeof the correct pointer in call to memset()

When clearing the ib_ah_attr parameter in to_ib_ah_attr(), use sizeof
*ib_ah_attr instead of sizeof *path.  This is the same bug as was
fixed for mthca in 99d4f22e

 ("IB/mthca: Use correct structure size in
call to memset()"), but the code was cut and pasted into mlx4 before the
fix was merged.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>