ext2: initialize opts.s_mount_opt as zero before using it

We need to initialize opts.s_mount_opt as zero before using it, else we
may get some unexpected mount options.

Fixes: 08851957 ("ext2: Parse mount options into a dedicated structure")
CC: stable@vger.kernel.org
Signed-off-by: xingaopeng <xingaopeng@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>