Use separate output buffers for OCF requests in KTLS.

KTLS encryption requests for file-backed data such as from sendfile(2)
require the encrypted data to be stored in a separate buffer from the
unencrypted file input data. Previously the OCF backend for KTLS
manually copied the data from the input buffer to the output buffer
before queueing the crypto request. Now the OCF backend will use a
separate output buffer for such requests and avoid the copy. This
mostly helps when an async co-processor is used by saving CPU cycles
used on the copy.

Reviewed by: gallatin (earlier version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24545