Improve length handling when writing sense data.

Improve length handling when writing sense data.

 - Allow maximal sense size limitation via Control Extension mode page.
 - When sense size limited, include descriptors atomically: whole or none.
 - Set new SDAT_OVFL bit if some descriptors don't fit the limit.
 - Report real written sense length instead of static maximal 252 bytes.

MFC after:	2 weeks