scsi: target: Add missing emulate_pr attribute to passthrough backends

scsi: target: Add missing emulate_pr attribute to passthrough backends

In commit f5b36e69a517 ("scsi: target: add emulate_pr backstore attr to
toggle PR support") the new attribute emulate_pr was added.

passthrough_parse_cdb() uses the attribute's value to distinguish whether
reservation commands should be rejected or not.  But the new attribute was
not added to passthrough_attrib_attrs, so in pscsi and tcmu - the users of
passthrough_parse_cdb() - the attribute is not available to change parser's
behavior.

Link: https://lore.kernel.org/r/20200427150823.15350-2-bstroesser@ts.fujitsu.com
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>