MFC r310524: Improve length handling when writing sense data.

MFC r310524: 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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@311440 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f