net: batch calls to flush_all_backlogs()

After commit 145dd5f9

 ("net: flush the softnet backlog in process
context"), we can easily batch calls to flush_all_backlogs() for all
devices processed in rollback_registered_many()

Tested:

Before patch, on an idle host.

modprobe dummy numdummies=10000
perf stat -e context-switches -a rmmod dummy

 Performance counter stats for 'system wide':

         1,211,798      context-switches

       1.302137465 seconds time elapsed

After patch:

perf stat -e context-switches -a rmmod dummy

 Performance counter stats for 'system wide':

           225,523      context-switches

       0.721623566 seconds time elapsed

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>