Merge branch 'seg6-fix-dst_cache-sharing-in-seg6-lwtunnel'

Andrea Mayer says:

====================
seg6: fix dst_cache sharing in seg6 lwtunnel

The seg6 lwtunnel encap uses a single per-route dst_cache shared
between seg6_input_core() and seg6_output_core(). These two paths
can perform the post-encap SID lookup in different routing contexts
(e.g., ip rules matching on the ingress interface, or VRF table
separation). Whichever path runs first populates the cache, and the
other reuses it blindly, bypassing its own lookup.

Patch 1 fixes this by splitting the cache into cache_input and
cache_output. Patch 2 adds a selftest that validates the isolation.
====================

Link: https://patch.msgid.link/20260404004405.4057-1-andrea.mayer@uniroma2.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>