IB/rxe: Prevent from completer to operate on non valid QP

On UD QP completer tasklet is scheduled for each packet sent.

If it is followed by a destroy_qp(), the kernel panic will
happen as the completer tries to operate on a destroyed QP.

Fixes: 8700e3e7

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