f2fs: use dget_parent and file_dentry in f2fs_file_open

This patch synced with the below two ext4 crypto fixes together.

In 4.6-rc1, f2fs newly introduced accessing f_path.dentry which crashes
overlayfs. To fix, now we need to use file_dentry() to access that field.

Fixes: c0a37d487884 ("ext4: use file_dentry()")
Fixes: 9dd78d8c9a7b ("ext4: use dget_parent() in ext4_file_open()")
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -441,7 +441,7 @@ static int f2fs_file_mmap(struct file *file, struct vm_area_struct *vma)
 static int f2fs_file_open(struct inode *inode, struct file *filp)
 {
 	int ret = generic_file_open(inode, filp);
-	struct inode *dir = filp->f_path.dentry->d_parent->d_inode;
+	struct dentry *dir;
 
 	if (!ret && f2fs_encrypted_inode(inode)) {
 		ret = fscrypt_get_encryption_info(inode);
@@ -450,9 +450,13 @@ static int f2fs_file_open(struct inode *inode, struct file *filp)
 		if (!fscrypt_has_encryption_key(inode))
 			return -ENOKEY;
 	}
-	if (f2fs_encrypted_inode(dir) &&
-			!fscrypt_has_permitted_context(dir, inode))
+	dir = dget_parent(file_dentry(filp));
+	if (f2fs_encrypted_inode(d_inode(dir)) &&
+			!fscrypt_has_permitted_context(d_inode(dir), inode)) {
+		dput(dir);
 		return -EPERM;
+	}
+	dput(dir);
 	return ret;
 }