net: ipv6: fix dst refleaks in rpl, seg6 and ioam6 lwtunnels

dst_cache_get() gives us a reference, we need to release it.

Discovered by the ioam6.sh test, kmemleak was recently fixed
to catch per-cpu memory leaks.

Fixes: 985ec6f5 ("net: ipv6: rpl_iptunnel: mitigate 2-realloc issue")
Fixes: 40475b63 ("net: ipv6: seg6_iptunnel: mitigate 2-realloc issue")
Fixes: dce52518 ("net: ipv6: ioam6_iptunnel: mitigate 2-realloc issue")
Reviewed-by: Justin Iurman <justin.iurman@uliege.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250130031519.2716843-1-kuba@kernel.org


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