fork: Stop allowing kthreads to call execve

Now that kernel_execve is no longer called from kernel threads stop
supporting kernel threads calling kernel_execve.

Remove the code for converting a kthread to a normal thread in execve.

Document the restriction that kthreads may not call kernel_execve by
having kernel_execve fail if called by a kthread.

Link: https://lkml.kernel.org/r/20220506141512.516114-7-ebiederm@xmission.comSigned-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1308,7 +1308,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out_unlock;
 
-	me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
+	me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC |
 					PF_NOFREEZE | PF_NO_SETAFFINITY);
 	flush_thread();
 	me->personality &= ~bprm->per_clear;
@@ -1953,8 +1953,8 @@ int kernel_execve(const char *kernel_filename,
 	int fd = AT_FDCWD;
 	int retval;
 
-	if (WARN_ON_ONCE((current->flags & PF_KTHREAD) &&
-			(current->worker_private)))
+	/* It is non-sense for kernel threads to call execve */
+	if (WARN_ON_ONCE(current->flags & PF_KTHREAD))
 		return -EINVAL;
 
 	filename = getname_kernel(kernel_filename);