Kernel: Fix a nasty lock bug with exec()

Exec doesn't leave through the syscall handler, so it didn't unlock the
big_lock. This means that reentering can lock it again, and then another
thread could endlessly yield waiting to acquire the lock (futilely).

This fixes AudioServer using 100% CPU.

Kernel/Process.cpp

@@ -442,6 +442,7 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
 #endif
 
     main_thread().set_state(Thread::State::Skip1SchedulerPass);
+    big_lock().unlock_if_locked();
     return 0;
 }