[CIFS] Explicitly set stat->blksize

CIFS may perform I/O over the network in larger chunks than the page size,
so it should explicitly set stat->blksize to ensure optimal I/O bandwidth

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>