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

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.

Cc: stable@vger.kernel.org
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>

drivers/target/target_core_user.c

@@ -686,8 +686,6 @@ static int tcmu_check_expired_cmd(int id, void *p, void *data)
 	target_complete_cmd(cmd->se_cmd, SAM_STAT_CHECK_CONDITION);
 	cmd->se_cmd = NULL;
 
-	kmem_cache_free(tcmu_cmd_cache, cmd);
-
 	return 0;
 }