usb: gadget: u_ether: add a flag to avoid skb_reserve() calling

This patch adds a flag "no_skb_reserve" in struct eth_dev.
So, if a peripheral driver sets the quirk_avoids_skb_reserve flag,
upper network gadget drivers (e.g. f_ncm.c) can avoid skb_reserve()
calling using the flag as well.

Signed-off-by: Yoshihiro Shimoda <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/function/u_ether.c

···
 #define	WORK_RX_MEMORY		0
 
 	bool			zlp;
+	bool			no_skb_reserve;
 	u8			host_mac[ETH_ALEN];
 	u8			dev_mac[ETH_ALEN];
 };
···
 	 * but on at least one, checksumming fails otherwise.  Note:
 	 * RNDIS headers involve variable numbers of LE32 values.
 	 */
-	skb_reserve(skb, NET_IP_ALIGN);
+	if (likely(!dev->no_skb_reserve))
+		skb_reserve(skb, NET_IP_ALIGN);
 
 	req->buf = skb->data;
 	req->length = size;
···
 
 	if (result == 0) {
 		dev->zlp = link->is_zlp_ok;
+		dev->no_skb_reserve = link->no_skb_reserve;
 		DBG(dev, "qlen %d\n", qlen(dev->gadget, dev->qmult));
 
 		dev->header_len = link->header_len;

drivers/usb/gadget/function/u_ether.h

···
 	struct usb_ep			*out_ep;
 
 	bool				is_zlp_ok;
+	bool				no_skb_reserve;
 
 	u16				cdc_filter;