In the MCTP route input test, we're routing one skb, then (when delivery
is expected) checking the resulting routed skb.

However, we're currently checking the original skb length, rather than
the routed skb. Check the routed skb instead; the original will have
been freed at this point.

Fixes: 8892c049 ("mctp: Add route input to socket tests")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/kernel-janitors/4ad204f0-94cf-46c5-bdab-49592addf315@kili.mountain/Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240816-mctp-kunit-skb-fix-v1-1-3c367ac89c27@codeconstruct.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>