selftests: net: tcp_mmap: fix SO_RCVLOWAT setting

Since chunk_size is no longer an integer, we can not
use it directly as an argument of setsockopt().

This patch should fix tcp_mmap for Big Endian kernels.

Fixes: 597b01ed

 ("selftests: net: avoid ptl lock contention in tcp_mmap")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>