oceani: propagate_type should return the found type, not what it was compatible with

oceani: propagate_type should return the found type, not what it was compatible with

If the type we have is different from the type expected, but they are
compatible, propagate_type() should return the type that we have.
Otherwise we lose information - which may later be useful.

Signed-off-by: NeilBrown <neil@brown.name>