nfp: flower: handle acti_netdevs allocation failure

The kmalloc_array() in nfp_fl_lag_do_work() will return null, if
the physical memory has run out. As a result, if we dereference
the acti_netdevs, the null pointer dereference bugs will happen.

This patch adds a check to judge whether allocation failure occurs.
If it happens, the delayed work will be rescheduled and try again.

Fixes: bb9a8d03

 ("nfp: flower: monitor and offload LAG groups")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Link: https://lore.kernel.org/r/20240308142540.9674-1-duoming@zju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>