net: ipv6: rpl_iptunnel: Fix memory leak in rpl_input

Free the skb before returning from rpl_input when skb_cow_head() fails.
Use a "drop" label and goto instructions.

Fixes: a7a29f9c ("net: ipv6: add rpl sr tunnel")
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240911174557.11536-1-justin.iurman@uliege.be


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