geli: enable direct dispatch

geli does all of its crypto operations in a separate thread pool, so
g_eli_start, g_eli_read_done, and g_eli_write_done don't actually do very
much work. Enabling direct dispatch eliminates the g_up/g_down bottlenecks,
doubling IOPs on my system. This change does not affect the thread pool.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D25587