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