From: zhenyu.z.wang@intel.com

Delay the head digest update until xmit time, like data digest update.
[To make things cleaner and avoid prempt bug]
Signed-off-by: NAlex Aizman <itn780@yahoo.com>
Signed-off-by: NDmitry Yusupov <dmitry_yus@yahoo.com>
Signed-off-by: NMike Christie <michaelc@cs.wisc.edu>
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>