ext3: force ro mount if ext3_setup_super() fails

If ext3_setup_super() fails i.e. due to a too-high revision,
the error is logged in dmesg but the fs is not mounted RO as
indicated.

Tested by:

[164152.114551] EXT3-fs (sdb6): error: revision level too high, forcing read-only mode
/dev/sdb6 /mnt/test2 ext3 rw,seclabel,relatime,errors=continue,user_xattr,acl,barrier=1,data=ordered 0 0
                          ^^

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Jan Kara <jack@suse.cz>