Currently, all xmit flows use dst_cache in a way that references
its dst_entry for each xmitted packet. These atomic operations
are redundant in some flows.

This patchset introduces new noref xmit helpers, and incorporates
them in the OpenVPN driver.

Changes in v2:
 - Fixed docstrings for dst_cache_get_ip4_rcu and dst_cache_steal_ip4

Link to v1: https://lore.kernel.org/netdev/20250909054333.12572-1-mmietus97@yahoo.com/ (local)
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2.51.0