selftests: mptcp: make sendfile selftest work

When the selftest got added, sendfile() on mptcp sockets returned
-EOPNOTSUPP, so running 'mptcp_connect.sh -m sendfile' failed
immediately.

This is no longer the case, but the script fails anyway due to timeout.
Let the receiver know once the sender has sent all data, just like
with '-m mmap' mode.

v2: need to respect cfg_wait too, as pm_userspace.sh relied
on -m sendfile to keep the connection open (Mat Martineau)

Fixes: 048d19d4

 ("mptcp: add basic kselftest for mptcp")
Reported-by: Xiumei Mu <xmu@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>