IB/srp: Fix unmapping of fake scatterlist

The recently merged patch to create a fake scatterlist for non-SG SCSI
commands had a bug: the driver ended up doing dma_unmap_sg() on a
scatterlist scmnd->request_buffer rather than the fake scatter list it
created.  Fix this so that the driver unmaps the same thing it maps.

Signed-off-by: Roland Dreier <rolandd@cisco.com>