net/fq_impl: drop get_default_func, move default flow to fq_tin

Simplifies the code and prepares for a rework of scanning for flows on
overmemory drop.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20201218184718.93650-2-nbd@nbd.name


Signed-off-by: Johannes Berg <johannes.berg@intel.com>