vxlan: Copy needed_tailroom from lowerdev

While vxlan doesn't need any extra tailroom, the lowerdev might need it. In
that case, copy it over to reduce the chance for additional (re)allocations
in the transmit path.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Link: https://lore.kernel.org/r/20201126125247.1047977-2-sven@narfation.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/vxlan.c

@@ -3800,6 +3800,8 @@ static void vxlan_config_apply(struct net_device *dev,
 		needed_headroom = lowerdev->hard_header_len;
 		needed_headroom += lowerdev->needed_headroom;
 
+		dev->needed_tailroom = lowerdev->needed_tailroom;
+
 		max_mtu = lowerdev->mtu - (use_ipv6 ? VXLAN6_HEADROOM :
 					   VXLAN_HEADROOM);
 		if (max_mtu < ETH_MIN_MTU)