Enable use of 4KB MTU.  Since the driver uses more pinned memory for
receive buffers when the 4KB MTU is enabled, whether or not the fabric
supports that MTU, add a "mtu4096" module parameter that can be used to
limit the MTU to 2KB when it is known that 4KB MTUs can't be used
anyway.
Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>