selftests: mptcp: connect: better display the files size

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.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-6-2ffb16a6cf35@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>