IB/rxe: Fix kmem_cache leak

Decrement qp reference when handling error path
in completer to prevent kmem_cache leak.

Fixes: 8700e3e7 ("Soft RoCE driver")
Signed-off-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>