NFS: Fix a number of sparse warnings

Fix a number of "warning: symbol 'foo' was not declared. Should it be
static?" conditions.

Fix 2 cases of "warning: Using plain integer as NULL pointer"

fs/nfs/delegation.c:263:31: warning: restricted fmode_t degrades to integer
  - We want to allow upgrades to a WRITE delegation, but should otherwise
    consider servers that hand out duplicate delegations to be borken.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pnfs_dev.c

@@ -92,7 +92,7 @@ _lookup_deviceid(const struct pnfs_layoutdriver_type *ld,
  * @clp nfs_client associated with deviceid
  * @id deviceid to look up
  */
-struct nfs4_deviceid_node *
+static struct nfs4_deviceid_node *
 _find_get_deviceid(const struct pnfs_layoutdriver_type *ld,
 		   const struct nfs_client *clp, const struct nfs4_deviceid *id,
 		   long hash)