init: Deal with the init process being a user mode process

It is silly for user_mode_thread to leave PF_KTHREAD set
on the resulting task.  Update the init process so that
it does not care if PF_KTHREAD is set or not.

Ensure do_populate_rootfs flushes all delayed fput work by calling
task_work_run.  In the rare instance that async_schedule_domain calls
do_populate_rootfs synchronously it is possible do_populate_rootfs
will be called directly from the init process.  At which point fput
will call "task_work_add(current, ..., TWA_RESUME)".  The files on the
initramfs need to be completely put before we attempt to exec them
(which is before the code enters userspace).  So call task_work_run
just in case there are any pending fput operations.

Link: https://lkml.kernel.org/r/20220506141512.516114-5-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

init/initramfs.c

@@ -15,6 +15,7 @@
 #include <linux/mm.h>
 #include <linux/namei.h>
 #include <linux/init_syscalls.h>
+#include <linux/task_work.h>
 #include <linux/umh.h>
 
 static ssize_t __init xwrite(struct file *file, const char *p, size_t count,
@@ -703,6 +704,7 @@ done:
 	initrd_end = 0;
 
 	flush_delayed_fput();
+	task_work_run();
 }
 
 static ASYNC_DOMAIN_EXCLUSIVE(initramfs_domain);