target: reject COMPARE_AND_WRITE if emulate_caw is not set

In struct se_dev_attrib, there is a field emulate_caw exposed
as a /sys/kernel/config/target/core/$HBA/$DEV/attrib/.

If this field is set zero, it means the corresponding struct se_device
does not support the scsi cmd COMPARE_AND_WRITE

In function sbc_parse_cdb(), go ahead and reject scsi COMPARE_AND_WRITE
if emulate_caw is not set, because it has been explicitly disabled
from user-space.

(Make pr_err ratelimited - nab)

Signed-off-by: Jiang Yi <jiangyilism@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>