docs: gdbmacros: print newest record

@head_id points to the newest record, but the printing loop
exits when it increments to this value (before printing).

Exit the printing loop after the newest record has been printed.

The python-based function in scripts/gdb/linux/dmesg.py already
does this correctly.

Fixes: e60768311a ("scripts/gdb: update for lockless printk ringbuffer")
Cc: stable@vger.kernel.org
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221229134339.197627-1-john.ogness@linutronix.de

Documentation/admin-guide/kdump/gdbmacros.txt

@@ -312,10 +312,10 @@ define dmesg
 			set var $prev_flags = $info->flags
 		end
 
-		set var $id = ($id + 1) & $id_mask
 		if ($id == $end_id)
 			loop_break
 		end
+		set var $id = ($id + 1) & $id_mask
 	end
 end
 document dmesg