ext2: Check block size validity during mount

[ Upstream commit 62aeb944 ]

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