selftests: mptcp: add option to specify size of file to transfer

The script generates two random files that are then sent via tcp and
mptcp connections.

In order to compare throughput over consecutive runs add an option
to provide the file size on the command line: "-f 128000".

Also add an option, -t, to enable tcp tests. This is useful to
compare throughput of mptcp connections and tcp connections.

Example: run tests with a 4mb file size, 300ms delay 0.01% loss,
default gso/tso/gro settings and with large write/blocking io:

mptcp_connect.sh -t -f $((4 * 1024 * 1024)) -d 300 -l 0.01%  -r 0 -e "" -m mmap

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>