ktls: move ktls_enable() within ktls_start()

ktls: move ktls_enable() within ktls_start()

On linux ktls can only be enabled on established TCP sockets.
When SSL_set_fd() is called before the connection is established
ktls_enable() fails and ktls is not setup.

This moves ktls_enable() call within then ktls_start() function.
Multiple calls to ktls_start() will trigger additional ktls_enable()
calls which fail with EEXIST, but do not affect the ktls socket.

CLA: trivial

Signed-off-by: Ulrich Weber <ulrich.weber@gmail.com>
Reviewed-by: Paul Yang <paulyang.inf@gmail.com>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27908)