Curently after the host sends a REPLACETLSPSK we free the TLS keys as
part of calling nvmet_auth_sq_free() on success. This means when the
host sends a follow up REPLACETLSPSK we return CONCAT_MISMATCH as the
check for !nvmet_queue_tls_keyid(req->sq) fails.

A previous attempt to fix this involed not calling nvmet_auth_sq_free()
on successful connections, but that results in memory leaks. Instead we
should not clear `tls_key` in nvmet_auth_sq_free(), as that was
incorrectly wiping the tls keys which are used for the session.

This patch ensures we correctly free the ephemeral session key on
connection, yet we don't free the TLS key unless closing the connection.

Signed-off-by: Alistair Francis <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Chris Leech <[email protected]>