selftests: mptcp: reuse linkfail to make given size files

This patch reused the test_linkfail values above 2 to make test files with
the given sizes (KB) for both the client side and the server side. It's
useful for the test cases using different file sizes.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>