ktls: Avoid wakeups and locking for synchronous callbacks

When performing encryption in software, the KTLS crypto callback always
locks the session to deliver a wakeup. But, if we're handling the
operation synchronously this is wasted effort and can result in
sleepqueue lock contention on large systems.

Use CRYPTO_SESS_SYNC() to determine whether the operation will be
completed asynchronously or not, and select a callback appropriately.
Avoid locking the session to check for completion if the session handles
requests synchronously.

Reviewed by: jhb
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28195