mxmount: Avoid mounting /amd/host/0 a second time

When booting from a usb stick, `/amd/mxonastick/0` is explicitly mounted
with a label other than data0. However, if there is an existing "data0"
filesystem (e.g. on the internal hdd), this is mounted on top of it.

There is code to avoid that in mxmounts, but it doesn't work because
`DEFAULT_MOUNT_PREFIX` contains `/amd/SHORTHOST` and so the path is
compared to `/amd/SHORTHOST/$hostname/0` instead of `/amd/$hostname/0`.

Fixes 6c9441e ("fix data0 destination
overmount")

mxmount/mxmount

@@ -72,7 +72,7 @@ sub add_data0_if_not_present {
         if ($mp->{label} eq "data0") {
             return;
         }
-        if ($mp->{mountpoint} eq "$V{DEFAULT_MOUNT_PREFIX}/$hostname/0") {
+        if ($mp->{mountpoint} eq "/amd/$hostname/0") {
             print STDERR "$mp->{mountpoint} already blocked by $mp->{label}\n";
             return;
         }