mlxsw: core: Create an ordered workqueue for FIB offload

We're going to start processing FIB entries addition / deletion events
in deferred work. These work items must be processed in the order they
were submitted or otherwise we can have differences between the kernel's
FIB table and the device's.

Solve this by creating an ordered workqueue to which these work items
will be submitted to. Note that we can't simply convert the current
workqueue to be ordered, as EMADs re-transmissions are also processed in
deferred work.

Later on, we can migrate other work items to this workqueue, such as FDB
notification processing and nexthop resolution, since they all take the
same lock anyway.

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