Add per-session locking to cryptosoft (swcr).

As part of ZFS Crypto, I started getting a series of panics when I did not
have AESNI loaded. Adding locking fixed it, and I concluded that the
Reinit function altered the AES key schedule. This locking is not as
fine-grained as it could be (AESNI uses per-cpu locking), but
it's minimally invasive.

Sponsored by: iXsystems Inc
Reviewed by: cem, mav
Approved by: re (gjb), mav (mentor)
Differential Revision: https://reviews.freebsd.org/D17307