ext2: Add more validity checks for inode counts

Add checks verifying number of inodes stored in the superblock matches
the number computed from number of inodes per group. Also verify we have
at least one block worth of inodes per group. This prevents crashes on
corrupted filesystems.

Reported-by:  <syzbot+d273f7d7f58afd93be48@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>