bnx2x: Prevent FW assertion when using Vxlan

FW has a rare corner case in which a fragmented packet using lots
of frags would not be linearized, causing the FW to assert while trying
to transmit the packet.

To prevent this, we need to make sure the window of fragements containing
MSS worth of data contains 1 BD less than for regular packets due to
the additional parsing BD.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>