net: selftests: initialize TCP header and skb payload with zero

Zero-initialize TCP header via memset() to avoid garbage values that
may affect checksum or behavior during test transmission.

Also zero-fill allocated payload and padding regions using memset()
after skb_put(), ensuring deterministic content for all outgoing
test packets.

Fixes: 3e1e58d6 ("net: add generic selftest support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250416160125.2914724-1-o.rempel@pengutronix.de


Signed-off-by: Paolo Abeni <pabeni@redhat.com>