kcrypto_aes: Use separate sessions for AES and SHA1

Some hardware supports AES acceleration but not SHA1, e.g., AES-NI
extensions. It is useful to have accelerated AES even if SHA1 must be
software.

Suggested by: asomers
Reviewed by: asomers, dfr
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5146