target/user: Fix use-after-free of tcmu_cmds if they are expired

commit d0905ca757bc40bd1ebc261a448a521b064777d7 upstream.

Don't free the cmd in tcmu_check_expired_cmd, it's still referenced by
an entry in our cmd_id->cmd idr. If userspace ever resumes processing,
tcmu_handle_completions() will use the now-invalid cmd pointer.

Instead, don't free cmd. It will be freed by tcmu_handle_completion() if
userspace ever recovers, or tcmu_free_device if not.

Reported-by: Bryant G Ly <bgly@us.ibm.com>
Tested-by: Bryant G Ly <bgly@us.ibm.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>