be less tricky about when ifq_free is handled.

be less tricky about when ifq_free is handled.

instead of assuming start routines only run inside the ifq serialiser,
only rely on the serialisation provided by the ifq mtx which is
explicitly used during ifq_deq ops.

ie, free the mbufs in ifq_free at the end of ifq_deq ops instead
of in the ifq_serialiser loop. ifq deq ops arent necessarily called
within the serialiser.

this should fix panics caused by fq codel on top of bce (which calls
bce_start from it's tx completion path instead of ifq_restart).

ok mikeb@