Implement sbp2_queue_work(), which is now a very simple accessor to one
of the struct sbp2_logical_unit members, right after the definition of
struct sbp2_logical_unit.

Put the sbp2_reconnect() implementation right after the sbp2_login()
implementation.  They are both part of the SBP-2 access protocol.

Implement the driver methods sbp2_probe(), spp2_update(), sbp2_remove()
in this order, reflecting the lifetime of an SBP-2 target.

Place the sbp2_release_target() implementation right next to
sbp2_remove() which is its primary user, and after sbp2_probe() which is
the counterpart to sbp2_release_target().

There are no changes to the implementations here, or at least not meant
to be.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>