selftests: mptcp: diag: use mptcp_lib_get_info_value

selftests: mptcp: diag: use mptcp_lib_get_info_value

When running diag.sh in a loop, chk_dump_one will report the following
"grep: write error":

 13 ....chk 2 cestab                                  [ OK ]
 grep: write error
 14 ....chk dump_one                                  [ OK ]
 15 ....chk 2->0 msk in use after flush               [ OK ]
 16 ....chk 2->0 cestab after flush                   [ OK ]

This error is caused by a broken pipe. When the output of 'ss' is processed
by grep, 'head -n 1' will exit immediately after getting the first line,
causing the subsequent pipe to close. At this time, if 'grep' is still
trying to write data to the closed pipe, it will trigger a SIGPIPE signal,
causing a write error.

One solution is not to use this problematic "head -n 1" command, but to use
mptcp_lib_get_info_value() helper defined in mptcp_lib.sh to get the value
of 'token'.

Fixes: ba2400166570 ("selftests: mptcp: add a test for mptcp_diag_dump_one")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Tested-by: Gang Yan <yangang@kylinos.cn>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250421-net-mptcp-pm-defer-freeing-v1-2-e731dc6e86b9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>