proc: Add nbcon support for /proc/consoles

Update /proc/consoles output to show 'W' if an nbcon console is
registered. Since the write_thread() callback is mandatory, it
enough just to check if it is an nbcon console.

Also update /proc/consoles output to show 'N' if it is an
nbcon console.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-14-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

fs/proc/consoles.c

@@ -21,6 +21,7 @@ static int show_console_dev(struct seq_file *m, void *v)
 		{ CON_ENABLED,		'E' },
 		{ CON_CONSDEV,		'C' },
 		{ CON_BOOT,		'B' },
+		{ CON_NBCON,		'N' },
 		{ CON_PRINTBUFFER,	'p' },
 		{ CON_BRL,		'b' },
 		{ CON_ANYTIME,		'a' },
@@ -58,8 +59,8 @@ static int show_console_dev(struct seq_file *m, void *v)
 	seq_printf(m, "%s%d", con->name, con->index);
 	seq_pad(m, ' ');
 	seq_printf(m, "%c%c%c (%s)", con->read ? 'R' : '-',
-			con->write ? 'W' : '-', con->unblank ? 'U' : '-',
-			flags);
+		   ((con->flags & CON_NBCON) || con->write) ? 'W' : '-',
+		   con->unblank ? 'U' : '-', flags);
 	if (dev)
 		seq_printf(m, " %4d:%d", MAJOR(dev), MINOR(dev));