[persistent-data, dm-array] add FIXME
Add method to register a low space callback to space maps.

FIXME: should we trigger the data sm low water map this way too?

Conflicts:
	drivers/md/dm-thin-metadata.c
	drivers/md/dm-thin-metadata.h
	drivers/md/persistent-data/dm-space-map-checker.c
	drivers/md/persistent-data/dm-space-map-metadata.c
dm-thin now sends an event when the metadata dev gets low
factor maybe_resize_data_dev out of pool_preresume

Conflicts:
	drivers/md/dm-thin.c
resize metadata stuff

Conflicts:
	drivers/md/dm-thin-metadata.c