Trivial fixed (doc, comments)
use wb->type in dmsetup table
change flag for migrate_mb(). need to add FUA flag since the caches re
after flushed.
remove FUA flag from migrate_buffer_mb
delete writeboost_resume
delete writeboost_merge

the bio is eventually splitted into 4KB cache blocks
dm-cache also doesn't implement .merge hook
modified tests

set max_active to 8 fo test parallel writes
not set io_min needless but left io_opt as it is

- add comment why we don't implement .merge
mv mutex_init to init_core() AND factor out setup_geom_info()