ext4: Add a WARN_ON_ONCE for querying LAST_IN_LEAF instead

We added the documentation in ext4_map_blocks() for usage of
EXT4_GET_BLOCKS_QUERY_LAST_IN_LEAF flag. But It's better to add
a WARN_ON_ONCE in case if anyone tries using this flag with CREATE to
avoid a random issue later. Since depth can change with CREATE and it
needs to be re-calculated before using it in there.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/ee6e82a224c50b432df9ce1ce3333c50182d8473.1747677758.git.ritesh.list@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: riteshharjani

fs/ext4/extents.c

@@ -4445,9 +4445,11 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	 * need to re-calculate the depth as it might have changed due to block
 	 * allocation.
 	 */
-	if (flags & EXT4_GET_BLOCKS_QUERY_LAST_IN_LEAF)
+	if (flags & EXT4_GET_BLOCKS_QUERY_LAST_IN_LEAF) {
+		WARN_ON_ONCE(flags & EXT4_GET_BLOCKS_CREATE);
 		if (!err && ex && (ex == EXT_LAST_EXTENT(path[depth].p_hdr)))
 			map->m_flags |= EXT4_MAP_QUERY_LAST_IN_LEAF;
+	}
 
 	ext4_free_ext_path(path);