bpf, sockmap: Update selftests to use skb_adjust_room

Instead of working around TLS headers in sockmap selftests use the
new skb_adjust_room helper. This allows us to avoid special casing
the receive side to skip headers.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/160160100932.7052.3646935243867660528.stgit@john-Precision-5820-Tower