net: ipv6: rpl_iptunnel: mitigate 2-realloc issue

This patch mitigates the two-reallocations issue with rpl_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
there is no impact (it even shows improvement):
- before: https://ibb.co/nQJhqwc
- after: https://ibb.co/4ZvW6wV



Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Cc: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>