nfs/blocklayout: add SCSI layout support

This is a trivial extension to the block layout driver to support the
new SCSI layouts draft.  There are three changes:

 - device identifcation through the SCSI VPD page.  This allows us to
   directly use the udev generated persistent device names instead of
   requiring an expensive lookup by crawling every block device node
   in /dev and reading a signature for it.
 - use of SCSI persistent reservations to protect device access and
   allow for robust fencing.  On the client sides this just means
   registering and unregistering a server supplied key.
 - an optimized LAYOUTCOMMIT payload that doesn't send unessecary
   fields to the server.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>