vxlan: Fix error that was resulting in VXLAN MTU size being 10 bytes too large

This change fixes an issue I found where VXLAN frames were fragmented when
they were up to the VXLAN MTU size.  I root caused the issue to the fact that
the headroom was 4 + 20 + 8 + 8.  This math doesn't appear to be correct
because we are not inserting a VLAN header, but instead a 2nd Ethernet header.
As such the math for the overhead should be 20 + 8 + 8 + 14 to account for the
extra headers that are inserted for VXLAN.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -50,8 +50,8 @@
 
 #define VXLAN_N_VID	(1u << 24)
 #define VXLAN_VID_MASK	(VXLAN_N_VID - 1)
-/* VLAN + IP header + UDP + VXLAN */
-#define VXLAN_HEADROOM (4 + 20 + 8 + 8)
+/* IP header + UDP + VXLAN + Ethernet header */
+#define VXLAN_HEADROOM (20 + 8 + 8 + 14)
 
 #define VXLAN_FLAGS 0x08000000	/* struct vxlanhdr.vx_flags required value. */