l2tp: Use inet_sk_init_flowi4() in l2tp_ip_sendmsg().

l2tp: Use inet_sk_init_flowi4() in l2tp_ip_sendmsg().

Use inet_sk_init_flowi4() to automatically initialise the flowi4
structure in l2tp_ip_sendmsg() instead of passing parameters manually
to ip_route_output_ports().

Override ->daddr with the value passed in the msghdr structure if
provided.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/2ff22a3560c5050228928456662b80b9c84a8fe4.1734357769.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>