GEOM: Set G_CF_DIRECT_SEND/RECEIVE for taste consumers.

All I/O requests through the taste consumers are synchronous, done
with g_read_data() and without any locks held. It makes no sense
to delegate the I/O to g_down/g_up threads.

This removes many of context switches during disk retaste.

MFC after: 2 weeks

(cherry picked from commit 10ae42ccbde4fe7c8c9de3d54f3c0d63457a1a68)