NFSv4: Fixes for nfs4_inode_return_delegation()

We mustn't call nfs_wb_all() on anything other than a regular file.
Furthermore, we can exit early when we don't hold a delegation.
Reported-by: David Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/delegation.c

@@ -755,11 +755,13 @@ int nfs4_inode_return_delegation(struct inode *inode)
 	struct nfs_delegation *delegation;
 
 	delegation = nfs_start_delegation_return(nfsi);
-	/* Synchronous recall of any application leases */
-	break_lease(inode, O_WRONLY | O_RDWR);
-	nfs_wb_all(inode);
-	if (delegation != NULL)
+	if (delegation != NULL) {
+		/* Synchronous recall of any application leases */
+		break_lease(inode, O_WRONLY | O_RDWR);
+		if (S_ISREG(inode->i_mode))
+			nfs_wb_all(inode);
 		return nfs_end_delegation_return(inode, delegation, 1);
+	}
 	return 0;
 }