ext2: convert to the new mount API

Convert ext2 to the new mount API.

Note that this makes the sb= option more accepting than it was before;
previosly, sb= was only accepted if it was the first specified option.
Now it can exist anywhere, and if respecified, the last specified value
is used.

Parse-time messages here are sent to ext2_msg with a NULL sb, and
ext2_msg is adjusted to accept that, as ext4 does today as well.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>