selftests: mptcp: connect: better display the files size

'du' will print the name of the file, which was already displayed
before, e.g.

  Created /tmp/tmp.UOyy0ghfmQ (size 4703740/tmp/tmp.UOyy0ghfmQ) containing data sent by client
  Created /tmp/tmp.xq3zvFinGo (size 1391724/tmp/tmp.xq3zvFinGo) containing data sent by server

'stat' can be used instead, to display this instead:

  Created /tmp/tmp.UOyy0ghfmQ (size 4703740 B) containing data sent by client
  Created /tmp/tmp.xq3zvFinGo (size 1391724 B) containing data sent by server

So easier to spot the file sizes.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -582,7 +582,7 @@ make_file()
 	mptcp_lib_make_file $name 1024 $ksize
 	dd if=/dev/urandom conv=notrunc of="$name" oflag=append bs=1 count=$rem 2> /dev/null
 
-	echo "Created $name (size $(du -b "$name")) containing data sent by $who"
+	echo "Created $name (size $(stat -c "%s" "$name") B) containing data sent by $who"
 }
 
 run_tests_lo()