net: accept an empty mask in /sys/class/net/*/queues/rx-*/rps_cpus

We must accept an empty mask in store_rps_map(), or we are not able
to disable RPS on a queue.

Fixes: 07bbecb3

 ("net: Restrict receive packets queuing to housekeeping CPUs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Maciej Żenczykowski <maze@google.com>
Cc: Alex Belits <abelits@marvell.com>
Cc: Nitesh Narayan Lal <nitesh@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Nitesh Narayan Lal <nitesh@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>