rocker: Implement FIB offload in deferred work

Convert rocker to offload FIBs in deferred work in a similar fashion to
mlxsw, which was converted in the previous commits.

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>