target/iscsi: Use proper SGL accessors for digest computation

target/iscsi: Use proper SGL accessors for digest computation

Current implementation assumes that all the buffers of an IO are linked
with a single SG list, which is OK because target-core is only allocating
a contigious scatterlist region.  However, this assumption is wrong for
se_cmd descriptors that want to use chaining across multiple SGL regions.

Fix this up by using proper SGL accessors for digest payload computation.

Signed-off-by: Alexei Potashnik <[email protected]>
Cc: Roland Dreier <[email protected]>
Signed-off-by: Nicholas Bellinger <[email protected]>