VFS: Handle lower layer dentry/inode in pathwalk

Make use of d_backing_inode() in pathwalk to gain access to an
inode or dentry that's on a lower layer.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/fs/open.c b/fs/open.c
index 98e5a52..e0250bd 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -367,7 +367,7 @@
 	if (res)
 		goto out;
 
-	inode = path.dentry->d_inode;
+	inode = d_backing_inode(path.dentry);
 
 	if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
 		/*