f2fs: prevent creating duplicate encrypted filenames

As described in "fscrypt: add fscrypt_is_nokey_name()", it's possible to
create a duplicate filename in an encrypted directory by creating a file
concurrently with adding the directory's encryption key.

Fix this bug on f2fs by rejecting no-key dentries in f2fs_add_link().

Note that the weird check for the current task in f2fs_do_add_link()
seems to make this bug difficult to reproduce on f2fs.

Fixes: 9ea97163c6 ("f2fs crypto: add filename encryption for f2fs_add_link")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/f2fs/f2fs.h

@@ -3251,6 +3251,8 @@ bool f2fs_empty_dir(struct inode *dir);
 
 static inline int f2fs_add_link(struct dentry *dentry, struct inode *inode)
 {
+	if (fscrypt_is_nokey_name(dentry))
+		return -ENOKEY;
 	return f2fs_do_add_link(d_inode(dentry->d_parent), &dentry->d_name,
 				inode, inode->i_ino, inode->i_mode);
 }