[SCSI] libosd: Fix bug in attr_page handling

The _osd_req_finalize_attr_page was off by a mile, when trying to
append the enc_get_attr segment instead of the proper set_attr segment.

Also properly support when we don't have any attribute to set while
getting a full page. And when clearing an attribute by setting it's
size to zero.

Reported-by: John Chandy <john.chandy@uconn.edu>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>