netpoll: factor out IPv6 header setup into push_ipv6() helper

Move IPv6 header construction from netpoll_send_udp() into a new
static helper function, push_ipv6(). This refactoring reduces code
duplication and improves readability in netpoll_send_udp().

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250702-netpoll_untagle_ip-v2-3-13cf3db24e2b@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>