f2fs: add sanity check on ei.len in __update_extent_tree_range()

Add a sanity check in __update_extent_tree_range() to detect any
zero-sized extent update.

Signed-off-by: wangzijie <wangzijie1@honor.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>