fs/proc: add Kthread flag to /proc/$pid/status

The command `ps -ef ` and `top -c` mark kernel thread by '[' and ']', but
sometimes the result is not correct.  The task->flags in /proc/$pid/stat
is good, but we need remember the value of PF_KTHREAD is 0x00200000 and
convert dec to hex.  If we have no binary program and shell script which
read /proc/$pid/stat, we can know it directly by `cat /proc/$pid/status`.

Link: https://lkml.kernel.org/r/20230416052404.2920-1-fullspring2018@gmail.com


Signed-off-by: Chunguang Wu <fullspring2018@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>