MFC 356507,356520: Add a reference count to cryptodev sessions.

356507:
Add a reference count to cryptodev sessions.

This prevents use-after-free races with crypto requests (which may
sleep) and CIOCFSESSION as well as races from current CIOCFSESSION
requests.

356520:
Remove no-longer-used function prototype.

admbugs: 949
Sponsored by: Chelsio Communications