net: sched: gred: allow manipulating per-DP RED flags

Allow users to set and dump RED flags (ECN enabled and harddrop)
on per-virtual queue basis.  Validation of attributes is split
from changes to make sure we won't have to undo previous operations
when we find out configuration is invalid.

The objective is to allow changing per-Qdisc parameters without
overwriting the per-vq configured flags.

Old user space will not pass the TCA_GRED_VQ_FLAGS attribute and
per-Qdisc flags will always get propagated to the virtual queues.

New user space which wants to make use of per-vq flags should set
per-Qdisc flags to 0 and then configure per-vq flags as it
sees fit.  Once per-vq flags are set per-Qdisc flags can't be
changed to non-zero.  Vice versa - if the per-Qdisc flags are
non-zero the TCA_GRED_VQ_FLAGS attribute has to either be omitted
or set to the same value as per-Qdisc flags.

Update per-Qdisc parameters:
per-Qdisc | per-VQ | result
        0 |      0 | all vq flags updated
	0 |  non-0 | error (vq flags in use)
    non-0 |      0 | -- impossible --
    non-0 |  non-0 | all vq flags updated

Update per-VQ state (flags parameter not specified):
   no change to flags

Update per-VQ state (flags parameter set):
per-Qdisc | per-VQ | result
        0 |   any  | per-vq flags updated
    non-0 |      0 | -- impossible --
    non-0 |  non-0 | error (per-Qdisc flags in use)

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>