pnfs/blocklayout: Don't add zero-length pnfs_block_dev

We noticed a SCSI device that refused to allow READ CAPACITY when the
device had a PR with exclusive access, registrants only.  The result of
this situation is that the blocklayout driver adds a pnfs_block_dev of zero
length which always fails the offset_in_map tests.  Instead of continuously
trying to do pNFS for this case, just mark the device as unavailable which
will allow the client to fallback to the MDS for the duration of
PNFS_DEVICE_RETRY_TIMEOUT.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>