net: ipv6: rpl_iptunnel: Fix potential memory leak in rpl_do_srh_inline

In case memory resources for buf were allocated, release them before
return.

Addresses-Coverity-ID: 1492011 ("Resource leak")
Fixes: a7a29f9c ("net: ipv6: add rpl sr tunnel")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>