ext2: Check block size validity during mount

Check that log of block size stored in the superblock has sensible
value. Otherwise the shift computing the block size can overflow leading
to undefined behavior.

Reported-by:  <syzbot+4fec412f59eba8c01b77@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>