selftests: netfilter: conntrack_ipip_mtu.sh: shellcheck cleanups

No functional change intended.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-9-fw@strlen.de


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