imsm: Use single migration type for all migrations

imsm: Use single migration type for all migrations

Use single enum definition/migration type for all migrations.
Using separate definitions causes limitation for number of changes
in metadata implementation during single update for migration/reshape.
Single CH_MIGRATION enum allows for many mtadata parameters change
in single update. It will be possible to change i.e. chunk size together
with raid level. In current implementation 2 metadata updates would be
required for such action, one using CH_CHUNK_MIGR and second using
CH_LEVEL_MIGRATION migration type.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>