selftests: net: test vxlan pmtu exceptions with tcp

TCP might get stuck if a nonlinear skb exceeds the path MTU,
icmp error contains an incorrect icmp checksum in that case.

Extend the existing test for vxlan to also send at least 1MB worth of
data via TCP in addition to the existing 'large icmp packet adds
route exception'.

On my test VM this fails due to 0-size output file without
"tunnels: fix kasan splat when generating ipv4 pmtu error".

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230803152653.29535-3-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>