cryptosoft: Reduce generality of supported algorithm composition

Fix a regression introduced in r336439.

Rather than allowing any linked list of algorithms, allow at most two
(typically, some combination of encrypt and/or MAC). Removes a WAITOK
malloc in an unsleepable context (classic LOR) by placing both software
algorithm contexts within the OCF-managed session object.

Tested with 'cryptocheck -a all -d cryptosoft0', which includes some
encrypt-and-MAC modes.

PR: 230304
Reported by: sef@