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.

admbugs: 949
Reported by: Yuval Kanarenstein <yuvalk@ssd-disclosure.com>
Reviewed by: cem
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D23077