net: ipv6: seg6_iptunnel: mitigate 2-realloc issue

net: ipv6: seg6_iptunnel: mitigate 2-realloc issue

[ Upstream commit 40475b63761abb6f8fdef960d03228a08662c9c4 ]

This patch mitigates the two-reallocations issue with seg6_iptunnel by
providing the dst_entry (in the cache) to the first call to
skb_cow_head(). As a result, the very first iteration would still
trigger two reallocations (i.e., empty cache), while next iterations
would only trigger a single reallocation.

Performance tests before/after applying this patch, which clearly shows
the improvement:
- before: https://ibb.co/3Cg4sNH
- after: https://ibb.co/8rQ350r

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Cc: David Lebrun <dlebrun@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 92191dd10730 ("net: ipv6: fix dst ref loops in rpl, seg6 and ioam6 lwtunnels")
Signed-off-by: Sasha Levin <sashal@kernel.org>