tcmu: fix cmd user after free

If we are failing the command due to a qfull timeout we are
also freeing the tcmu command, so we cannot access it later
to get the se_cmd.

Note: The clearing of cmd->se_cmd is not needed. We do not check
it later for something like determining if the command was failed
due to a timeout. As a result I am dropping it.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>