[SCSI] libiscsi: use get_unaligned

Dave Miller meantioned that the data buffer in a past
sense fixup patch was not gauranteed to be aligned
properly for ia64. This patch has libiscsi use get_unalinged
to make sure. There are a couple more places in the
digest handling we may need to do this, but we are in the middle
of fixing that code for big endien systems so just the sense
access is fixed here.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>