ext4: add check for wraparound in ext4_data_block_valid()

Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/block_validity.c

@@ -228,6 +228,7 @@ int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,
 	struct rb_node *n = sbi->system_blks.rb_node;
 
 	if ((start_blk <= le32_to_cpu(sbi->s_es->s_first_data_block)) ||
+	    (start_blk + count < start_blk) ||
 	    (start_blk + count > ext4_blocks_count(sbi->s_es)))
 		return 0;
 	while (n) {