Fix downgrading of TOE TLS sockets to plain TOE.

If a TOE TLS socket ends up using an unsupported TLS version or
ciphersuite, it must be downgraded to a "plain" TOE socket with TLS
encryption/decryption performed on the host. The previous
implementation of this fallback was incomplete and resulted in hung
connections.

Reviewed by: np
MFC after: 2 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D27467