selftests: net: switch to socat in the GSO GRE test

selftests: net: switch to socat in the GSO GRE test

Commit 89cdce434a0a ("selftests: net: properly support IPv6 in GSO GRE test")
is not compatible with:

  Ncat: Version 7.80 ( https://nmap.org/ncat )

(which is distributed with Fedora/Red Hat), tests fail with:

  nc: invalid option -- 'N'

Let's switch to socat which is far more dependable.

Fixes: 628f434d2129 ("selftests: add simple GSO GRE test")
Fixes: 89cdce434a0a ("selftests: net: properly support IPv6 in GSO GRE test")
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Link: https://lore.kernel.org/r/20211111162929.530470-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>