Bug 1065 (http://bugme.osdl.org/show_bug.cgi?id=1065) points out that there
is a risk that UDF will accidentally mount a reiserfs partition, which would
prevent a successful boot.

Andries points out that probing is unreliable and that users/admins should
always specify the root filesystem type on the kernel boot command line with
"rootfstype=resierfs".

But nobody does that, so we should be trying reiserfs before UDF.