ipc: avoid double grant entry

Check if the app is using IPC incorrectly and trying to notify itself.
If it is we need to return early so we do not double enter a grant.

kernel/src/ipc.rs

@@ -73,6 +73,13 @@ impl<const NUM_PROCS: u8> IPC<NUM_PROCS> {
     ) -> Result<(), process::Error> {
         let schedule_on_id = schedule_on.index().ok_or(process::Error::NoSuchApp)?;
         let called_from_id = called_from.index().ok_or(process::Error::NoSuchApp)?;
+
+        // Verify that IPC is not trying to share with the same app. If so, this
+        // will cause a double grant enter if we don't return now.
+        if schedule_on_id == called_from_id {
+            return Err(process::Error::AlreadyInUse);
+        }
+
         self.data.enter(schedule_on, |_, schedule_on_data| {
             self.data.enter(called_from, |_, called_from_data| {
                 // If the other app shared a buffer with us, make