pnfs/blocklayout: pass proper file mode to blkdev_get/put

We generally want to read and write to a block device that's used by
the pNFS block layout client (and even if it's read only the server
has no way of telling us).  Add FMODE_WRITE to the mode argument
so that we don't incorrectly tell the block driver that we want a
read-only open.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>