net: move devres helpers into a separate source file

There's currently only a single devres helper in net/ - devm variant
of alloc_etherdev. Let's move it to net/devres.c with the intention of
assing a second one: devm_register_netdev(). This new routine will need
to know the address of the release function of devm_alloc_etherdev() so
that it can verify (using devres_find()) that the struct net_device
that's being passed to it is also resource managed.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/devres.c

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * This file contains all networking devres helpers.
+ */
+
+#include <linux/device.h>
+#include <linux/etherdevice.h>
+#include <linux/netdevice.h>
+
+static void devm_free_netdev(struct device *dev, void *res)
+{
+	free_netdev(*(struct net_device **)res);
+}
+
+struct net_device *devm_alloc_etherdev_mqs(struct device *dev, int sizeof_priv,
+					   unsigned int txqs, unsigned int rxqs)
+{
+	struct net_device **dr;
+	struct net_device *netdev;
+
+	dr = devres_alloc(devm_free_netdev, sizeof(*dr), GFP_KERNEL);
+	if (!dr)
+		return NULL;
+
+	netdev = alloc_etherdev_mqs(sizeof_priv, txqs, rxqs);
+	if (!netdev) {
+		devres_free(dr);
+		return NULL;
+	}
+
+	*dr = netdev;
+	devres_add(dev, dr);
+
+	return netdev;
+}
+EXPORT_SYMBOL(devm_alloc_etherdev_mqs);