scsi-disk: support WRITE SAME (16) with unmap bit

Support discards via the WRITE SAME command with the unmap bit set, and
tell the initiator about the support for it via the block limit and the
new thin provisioning EVPD pages.  Also fix the comment which incorrectly
describedthe block limits EVPD page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
