pnfs: add a common GETDEVICELIST implementation

At a simple helper to issue a GETDEVICELIST operation and pre-load
the device id cache based on the result.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index ce89ae3..3eeca49 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -277,6 +277,8 @@
 void nfs4_mark_deviceid_unavailable(struct nfs4_deviceid_node *node);
 bool nfs4_test_deviceid_unavailable(struct nfs4_deviceid_node *node);
 void nfs4_deviceid_purge_client(const struct nfs_client *);
+int nfs4_deviceid_getdevicelist(struct nfs_server *server,
+		const struct nfs_fh *fh);
 
 static inline struct pnfs_layout_segment *
 pnfs_get_lseg(struct pnfs_layout_segment *lseg)