target/sbc: Fix sbc_dif_verify inconsistent map/unmap

Fix map/unmap consistency and get rid of a redundant
local variable psg.

Reported-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>