pnfs: ask for layout_blksize and save it in nfs_server

Block layout needs it to determine IO size.

Signed-off-by: Fred Isaman <iisaman@citi.umich.edu>
Signed-off-by: Tao Guo <glorioustao@gmail.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>