lwip reports via getsockopt the size of the default size of the receive buffer
to the netperf server. lwip returns 2GB and netperf server uses this value to
allocate some buffers - which of course fails with out of memory.
Reduces the "default size" to some smaller value.
With the commit we are not forced anymore to (but still can) use specific
netperf client options regarding memory allocations of the receive buffer.
8d9e7d0da2