dm log: move dirty region log code into separate module

Move the dirty region log code into a separate module so
other targets can share the code.

Signed-off-by: Heinz Mauelshagen <hjm@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/Makefile
Simple merge

diff --cc drivers/md/dm-log.c
Simple merge

diff --cc drivers/md/dm-raid1.c
Simple merge