nfp: move indirect block cleanup to flower app stop callback

The indirect block cleanup may cause control messages to be sent
if offloaded flows are present. However, by the time the flower app
cleanup callback is called txbufs are no longer available and attempts
to send control messages result in a NULL-pointer dereference in
nfp_ctrl_tx_one().

This problem may be resolved by moving the indirect block cleanup
to the stop callback, where txbufs are still available.

As suggested by Jakub Kicinski and Louis Peens.

Fixes: a1db2178

 ("net: flow_offload: fix flow_indr_dev_unregister path")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Louis Peens <louis.peens@netronome.com>
Link: https://lore.kernel.org/r/20201216145701.30005-1-simon.horman@netronome.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>