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>