net: micro-optimize skb_datagram_iter

We only use the mapping in a single context in a short and contained scope,
so kmap_local_page is sufficient and cheaper. This will also allow
skb_datagram_iter to be called from softirq context.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20240613113504.1079860-1-sagi@grimberg.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>