exportfs: add methods for block layout exports

Add three methods to allow exporting pnfs block layout volumes:

 - get_uuid: get a filesystem unique signature exposed to clients
 - map_blocks: map and if nessecary allocate blocks for a layout
 - commit_blocks: commit blocks in a layout once the client is done with them

For now we stick the external pnfs block layout interfaces into s_export_op to
avoid mixing them up with the internal interface between the NFS server and
the layout drivers.  Once we've fully internalized the latter interface we
can redecide if these methods should stay in s_export_ops.

Signed-off-by: Christoph Hellwig <hch@lst.de>