ext2: code cleanup for ext2_free_blocks()

Call ext2_data_block_valid() for block range validity.

Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Link: https://lore.kernel.org/r/20190723112155.20329-2-cgxu519@zoho.com.cn


Signed-off-by: Jan Kara <jack@suse.cz>