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.

Signed-off-by Chunguang Wu <[email protected]>

---
 Documentation/filesystems/proc.rst | 2 ++
 fs/proc/array.c                    | 2 ++
 2 files changed, 4 insertions(+)

Documentation/filesystems/proc.rst +2 -0

index 9d5fd9424e8b..8a563684586c 100644
@@ -179,6 +179,7 @@ read the file /proc/PID/status::
   Gid:    100     100     100     100
   FDSize: 256
   Groups: 100 14 16
+  Kthread:    0
   VmPeak:     5004 kB
   VmSize:     5004 kB
   VmLck:         0 kB

@@ -256,6 +257,7 @@ It's slow but very precise.
  NSpid                       descendant namespace process ID hierarchy
  NSpgid                      descendant namespace process group ID hierarchy
  NSsid                       descendant namespace session ID hierarchy
+ Kthread                     kernel thread flag, 1 is yes, 0 is no
  VmPeak                      peak virtual memory size
  VmSize                      total program size
  VmLck                       locked memory size

fs/proc/array.c +2 -0

index 9b0315d34c58..425824ad85e1 100644
@@ -219,6 +219,8 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		seq_put_decimal_ull(m, "\t", task_session_nr_ns(p, pid->numbers[g].ns));
 #endif
 	seq_putc(m, '\n');
+
+	seq_printf(m, "Kthread:\t%c\n", p->flags & PF_KTHREAD ? '1' : '0');
 }
 
 void render_sigset_t(struct seq_file *m, const char *header,