mlxsw: spectrum_qdisc: Support offloading of FIFO Qdisc

There are two peculiarities about offloading FIFO:

- sometimes the qdisc has an unspecified handle (it is "invisible")
- it may be created before the qdisc that it will be a child of

These features make the offload a bit more tricky. The approach chosen in
this patch is to make note of all the FIFOs that needed to be rejected
because their parents were not known. Later when the parent is created,
they are offloaded

FIFO is only offloaded for its counters, queue length is ignored.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>