Stop enforcing additional reference on all cdevs, which was introduced
in r277199. Acquire the neccessary reference in delist_dev_locked()
and inform destroy_devl() about it using CDP_UNREF_DTR flag.

Fix some style nits, add asserts.

Discussed with: hselasky
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week