ext2fs_open2: Add more checks for filesystem consistency

ext2fs_open2: Add more checks for filesystem consistency

Add more checks to assure the filesystem is sane to avoid later
division by zero errors.

Patch adds consistency checks on superblock to fix serveral crashes:
* require EXT2_INODES_PER_GROUP != 0
* require EXT2_DESC_PER_BLOCK != 0
* require s_first_data_block < s_blocks_count
* require group_desc_count * EXT2_INODES_PER_GROUP == s_inodes_count

Signed-off-by: Thiemo Nagel <thiemo.nagel@ph.tum.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>