selftests: net: tcp_mmap: Use huge pages in send path

There are significant gains using huge pages when
available, as shown in [1].

This patch adds mmap_large_buffer() and uses it
in client side (tx path of this reference tool)

Following patch will use the feature for server side.

[1] https://patchwork.ozlabs.org/project/netdev/patch/20200820154359.1806305-1-edumazet@google.com/

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>