brcmfmac: Don't grow SKB by negative size

The commit to rework the headroom check in start_xmit() now calls
pxskb_expand_head() unconditionally if the header is CoW. Unfortunately,
it does so with the delta between the extant headroom and the header
length, which may be negative if there is already sufficient headroom.

pskb_expand_head() does allow for size being 0, in which case it just
copies, so clamp the header delta to zero.

Opening Chrome (and all my tabs) on a PCIE device was enough to reliably
hit this.

Fixes: 270a6c1f65 ("brcmfmac: rework headroom check in .start_xmit()")
Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Arend Van Spriel <arend.vanspriel@broadcom.com>
Cc: James Hughes <james.hughes@raspberrypi.org>
Cc: Hante Meuleman <hante.meuleman@broadcom.com>
Cc: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Cc: Franky Lin <franky.lin@broadcom.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c

@@ -214,7 +214,7 @@ static netdev_tx_t brcmf_netdev_start_xmit(struct sk_buff *skb,
 
 	/* Make sure there's enough writeable headroom */
 	if (skb_headroom(skb) < drvr->hdrlen || skb_header_cloned(skb)) {
-		head_delta = drvr->hdrlen - skb_headroom(skb);
+		head_delta = max_t(int, drvr->hdrlen - skb_headroom(skb), 0);
 
 		brcmf_dbg(INFO, "%s: insufficient headroom (%d)\n",
 			  brcmf_ifname(ifp), head_delta);