ext3, ext4: avoid divide by zero

As it turns out, the kernel divides by EXT3_INODES_PER_GROUP(s) when
mounting an ext3 filesystem. If that number is zero, a crash follows.
Below a patch.

This crash was reported by Joeri de Ruiter, Carst Tankink and Pim Vullers.

Cc: <[e-mail]>
Acked-by: Alan Cox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ext3/super.c

···
 	sbi->s_blocks_per_group = le32_to_cpu(es->s_blocks_per_group);
 	sbi->s_frags_per_group = le32_to_cpu(es->s_frags_per_group);
 	sbi->s_inodes_per_group = le32_to_cpu(es->s_inodes_per_group);
-	if (EXT3_INODE_SIZE(sb) == 0)
+	if (EXT3_INODE_SIZE(sb) == 0 || EXT3_INODES_PER_GROUP(sb) == 0)
 		goto cantfind_ext3;
 	sbi->s_inodes_per_block = blocksize / EXT3_INODE_SIZE(sb);
 	if (sbi->s_inodes_per_block == 0)

fs/ext4/super.c

···
 		sbi->s_desc_size = EXT4_MIN_DESC_SIZE;
 	sbi->s_blocks_per_group = le32_to_cpu(es->s_blocks_per_group);
 	sbi->s_inodes_per_group = le32_to_cpu(es->s_inodes_per_group);
-	if (EXT4_INODE_SIZE(sb) == 0)
+	if (EXT4_INODE_SIZE(sb) == 0 || EXT4_INODES_PER_GROUP(sb) == 0)
 		goto cantfind_ext4;
 	sbi->s_inodes_per_block = blocksize / EXT4_INODE_SIZE(sb);
 	if (sbi->s_inodes_per_block == 0)