net sched actions: dump more than TCA_ACT_MAX_PRIO actions per batch

When you dump hundreds of thousands of actions, getting only 32 per
dump batch even when the socket buffer and memory allocations allow
is inefficient.

With this change, the user will get as many as possibly fitting
within the given constraints available to the kernel.

The top level action TLV space is extended. An attribute
TCA_ROOT_FLAGS is used to carry flags; flag TCA_FLAG_LARGE_DUMP_ON
is set by the user indicating the user is capable of processing
these large dumps. Older user space which doesnt set this flag
doesnt get the large (than 32) batches.
The kernel uses the TCA_ROOT_COUNT attribute to tell the user how many
actions are put in a single batch. As such user space app knows how long
to iterate (independent of the type of action being dumped)
instead of hardcoded maximum of 32 thus maintaining backward compat.

Some results dumping 1.5M actions below:
first an unpatched tc which doesnt understand these features...

prompt$ time -p tc actions ls action gact | grep index | wc -l
1500000
real 1388.43
user 2.07
sys 1386.79

Now lets see a patched tc which sets the correct flags when requesting
a dump:

prompt$ time -p updatedtc actions ls action gact | grep index | wc -l
1500000
real 178.13
user 2.02
sys 176.96

That is about 8x performance improvement for tc app which sets its
receive buffer to about 32K.

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>