ext4: fix use-after-iput when fscrypt contexts are inconsistent

In the case where the child's encryption context was inconsistent with
its parent directory, we were using inode->i_sb and inode->i_ino after
the inode had already been iput().  Fix this by doing the iput() in the
correct places.

Note: only ext4 had this bug, not f2fs and ubifs.

Fixes: d9cdc9033181 ("ext4 crypto: enforce context consistency")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/namei.c

@@ -1618,13 +1618,15 @@ static struct dentry *ext4_lookup(struct inode *dir, struct dentry *dentry, unsi
 		    !fscrypt_has_permitted_context(dir, inode)) {
 			int nokey = ext4_encrypted_inode(inode) &&
 				!fscrypt_has_encryption_key(inode);
-			iput(inode);
-			if (nokey)
+			if (nokey) {
+				iput(inode);
 				return ERR_PTR(-ENOKEY);
+			}
 			ext4_warning(inode->i_sb,
 				     "Inconsistent encryption contexts: %lu/%lu",
 				     (unsigned long) dir->i_ino,
 				     (unsigned long) inode->i_ino);
+			iput(inode);
 			return ERR_PTR(-EPERM);
 		}
 	}