IB/hns: fix memory leak on ah on error return path

When dmac is NULL, ah is not being freed on the error return path. Fix
this by kfree'ing it.

Detected by CoverityScan, CID#1452636 ("Resource Leak")

Fixes: d8966fcd ("IB/core: Use rdma_ah_attr accessor functions")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>