misc: fastrpc: return -EPIPE to invocations on device removal

The return value is initialized as -1, or -EPERM. The completion of an
invocation implies that the return value is set appropriately, but
"Permission denied" does not accurately describe the outcome of the
invocation. Set the invocation's return value to a more appropriate
"Broken pipe", as the cleanup breaks the driver's connection with rpmsg.

Fixes: c68cfb7 ("misc: fastrpc: Add support for context Invoke method")
Cc: stable <stable@kernel.org>
Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230523152550.438363-4-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -2349,8 +2349,10 @@ static void fastrpc_notify_users(struct fastrpc_user *user)
 	struct fastrpc_invoke_ctx *ctx;
 
 	spin_lock(&user->lock);
-	list_for_each_entry(ctx, &user->pending, node)
+	list_for_each_entry(ctx, &user->pending, node) {
+		ctx->retval = -EPIPE;
 		complete(&ctx->work);
+	}
 	spin_unlock(&user->lock);
 }