IB/core: Fix calculation of maximum RoCE MTU

The original code only took into consideration the largest header
possible after the IB_BTH_BYTES.  This was incorrect, as the largest
possible header size is the largest possible combination of headers we
might run into.  The new code accounts for all possible headers in the
largest possible combination and subtracts that from the MTU to make
sure that all packets will fit on the wire.

Link: https://www.spinics.net/lists/linux-rdma/msg54558.html
Fixes: 3c86aa70 ("RDMA/cm: Add RDMA CM support for IBoE devices")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Reported-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>

include/rdma/ib_addr.h

@@ -245,10 +245,11 @@ static inline void rdma_addr_set_dgid(struct rdma_dev_addr *dev_addr, union ib_g
 static inline enum ib_mtu iboe_get_mtu(int mtu)
 {
 	/*
-	 * reduce IB headers from effective IBoE MTU. 28 stands for
-	 * atomic header which is the biggest possible header after BTH
+	 * Reduce IB headers from effective IBoE MTU.
 	 */
-	mtu = mtu - IB_GRH_BYTES - IB_BTH_BYTES - 28;
+	mtu = mtu - (IB_GRH_BYTES + IB_UDP_BYTES + IB_BTH_BYTES +
+		     IB_EXT_XRC_BYTES + IB_EXT_ATOMICETH_BYTES +
+		     IB_ICRC_BYTES);
 
 	if (mtu >= ib_mtu_enum_to_int(IB_MTU_4096))
 		return IB_MTU_4096;

include/rdma/ib_pack.h

@@ -37,14 +37,17 @@
 #include <uapi/linux/if_ether.h>
 
 enum {
-	IB_LRH_BYTES  = 8,
-	IB_ETH_BYTES  = 14,
-	IB_VLAN_BYTES = 4,
-	IB_GRH_BYTES  = 40,
-	IB_IP4_BYTES  = 20,
-	IB_UDP_BYTES  = 8,
-	IB_BTH_BYTES  = 12,
-	IB_DETH_BYTES = 8
+	IB_LRH_BYTES		= 8,
+	IB_ETH_BYTES		= 14,
+	IB_VLAN_BYTES		= 4,
+	IB_GRH_BYTES		= 40,
+	IB_IP4_BYTES		= 20,
+	IB_UDP_BYTES		= 8,
+	IB_BTH_BYTES		= 12,
+	IB_DETH_BYTES		= 8,
+	IB_EXT_ATOMICETH_BYTES	= 28,
+	IB_EXT_XRC_BYTES	= 4,
+	IB_ICRC_BYTES		= 4
 };
 
 struct ib_field {