target: push session reinstatement out of transport_generic_free_cmd

Push session reinstatement out of transport_generic_free_cmd into the only
caller that actually needs it.  Clean up transport_generic_free_cmd a bit,
and remove the useless comment.  I'd love to add a more useful kerneldoc
comment for it, but as this point I'm still a bit confused in where it
stands in the command release stack.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>