ext4: use file_dentry()

EXT4 may be used as lower layer of overlayfs and accessing f_path.dentry
can lead to a crash.

Fix by replacing direct access of file->f_path.dentry with the
file_dentry() accessor, which will always return a native object.

Reported-by: Daniel Axtens <dja@axtens.net>
Fixes: 4bacc9c9234c ("overlayfs: Make f_path always point to the overlay and f_inode to the underlay")
Fixes: ff978b09f973 ("ext4 crypto: move context consistency check to ext4_file_open()")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org> # v4.5

fs/ext4/file.c

@@ -374,7 +374,7 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
 			return -ENOKEY;
 	}
 
-	dir = dget_parent(filp->f_path.dentry);
+	dir = dget_parent(file_dentry(filp));
 	if (ext4_encrypted_inode(d_inode(dir)) &&
 	    !ext4_is_child_context_consistent_with_parent(d_inode(dir), inode)) {
 		ext4_warning(inode->i_sb,