syscall: Add description of unavailable since v6.9

Close #270

Author: linD026

lkmpg.tex

@@ -1415,6 +1415,8 @@ \section{System Calls}
 But what if you want to do something unusual, to change the behavior of the system in some way?
 Then, you are mostly on your own.
 
+Notice that this example has been unavailable since v6.9, specifically after this \href{https://github.com/torvalds/linux/commit/1e3ad78334a69b36e107232e337f9d693dcc9df2#diff-4a16bf89a09b4f49669a30d54540f0b936ea0224dc6ee9edfa7700deb16c3e11R52}{commit}, due to the system call table changing the implementation from an indirect function call table to a switch statement.
+
 Should one choose not to use a virtual machine, kernel programming can become risky.
 For example, while writing the code below, the \cpp|open()| system call was inadvertently disrupted.
 This resulted in an inability to open any files, run programs, or shut down the system, necessitating a restart of the virtual machine.