free_pd is allocated internally by the driver hence needs to be freed
internally too or it leaks.

Fixes: 21a428a0 ("RDMA: Handle PD allocations by IB/core")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>