ext2: Add more validity checks for inode counts

[ Upstream commit fa78f336937240d1bc598db817d638086060e7e9 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>