new helper: file_inode(file)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 043ddcc..ade146b 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -625,7 +625,7 @@
 			return -EBUSY;
 		file = fget(arg);
 		if (file) {
-			struct inode *inode = file->f_path.dentry->d_inode;
+			struct inode *inode = file_inode(file);
 			if (S_ISSOCK(inode->i_mode)) {
 				nbd->file = file;
 				nbd->sock = SOCKET_I(inode);