glabel: Set G_CF_DIRECT_SEND/RECEIVE for taste consumer.

All I/O requests through the taste consumer 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 29998bf2ac4cc3ac49827336655bdbcb755eeffc)