IB/rxe: Fix a memory leak in rxe_qp_cleanup()

A socket is associated with every QP by the rxe driver but sock_release()
is never called. Add a call to sock_release() in rxe_qp_cleanup().

Fixes: commit 8700e3e7c48A5 ("Add Soft RoCE driver")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Moni Shoua <monis@mellanox.com>
Cc: Kamal Heib <kamalh@mellanox.com>
Cc: Amir Vadai <amirv@mellanox.com>
Cc: Haggai Eran <haggaie@mellanox.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>