[SCSI] libiscsi: don't bugon when if user sets markers

libiscsi does not support markers and if someone tries
to set them the driver does a BUG(). There is not need
to be that extreme. Just return -ENOSYS.

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