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: d8966fcd4c25 ("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>