dm-log: make dm-log.c a seperate sharable module

Hi Alasdair,

This is a series of 4 patches, aiming to support the replicator target by exporting dm_disk from dm.c and factoring the dirty region log (aka "mirror log" so far) out into a seperate kmod sharable by the mirror and the replicator target (and others in the future).

If acceptable, please apply.

Regards,
Heinz