RDMA/cxgb4: Don't free chunk that we have failed to allocate

In the error path of registering memory when there's a failure to
allocate a chunk from the memory pool, we try to free the same chunk
we just failed to allocate, which will BUG().

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>