net: Add NETIF_F_FCOE_MTU to indicate support for a different MTU for FCoE

Add NETIF_F_FCOE_MTU to indicate that the NIC can support a secondary MTU for
converged traffic of LAN and Fiber Channel over Ethernet (FCoE). The MTU for
FCoE is 2158 = 14 (FCoE header) + 24 (FC header) + 2112 (FC max payload) +
4 (FC CRC) + 4 (FCoE trailer).

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>