[SCSI] fcoe: Add sysfs parameter to fcoe for minimum DDP read I/O size

This adds fcoe_ddp_min as a module parameter for fcoe module to:
/sys/module/fcoe/parameters/ddp_min

It is observed that for some hardware, particularly Intel 82599, there is too
much overhead in setting up context for direct data placement (DDP) read when
the requested read I/O size is small. This is added as a module parameter for
performance tuning and is set as 0 by default and user can change this based
on their own hardware.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>