[SCSI] libfcoe: Do not pad FIP keep-alive to full frame size

According to the FC-BB-5 Rev2.0, 7.8.6.2, we should not pad FIP keep-alive
frames.

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>