net: ipv6: fix missing dst ref drop in ila lwtunnel

Add missing skb_dst_drop() to drop reference to the old dst before
adding the new dst to the skb.

Fixes: 79ff2fc3 ("ila: Cache a route to translated address")
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20250305081655.19032-1-justin.iurman@uliege.be


Signed-off-by: Paolo Abeni <pabeni@redhat.com>