GEOM: Relax direct dispatch for GEOM threads.

The only cases when direct dispatch does not make sense is for I/O
submission from down thread and for completion from up thread. In
all other cases, if both consumer and producer are OK about it, we
can save on context switches.

MFC after: 2 weeks

(cherry picked from commit ffc1cc95e78ec05a3e1a0aed869e33a44d9f6641)