selftests: mptcp: fix a mp_fail test warning

selftests: mptcp: fix a mp_fail test warning

Old tc versions (iproute2 5.3) show actions in multiple lines, not a
single line. Then the following unexpected MP_FAIL selftest output
occurs:

 file received by server has inverted byte at 169
 ./mptcp_join.sh: line 1277: [: [{"total acts":1},{"actions":[{"order":0 pedit ,"control_action":{"type":"pipe"}keys 1
         index 1 ref 1 bind 1,"installed":0,"last_used":0
         key #0  at 148: val ff000000 mask ffffffff
 5: integer expression expected
 001 Infinite map                      syn[ ok ] - synack[ ok ] - ack[ ok ]
                                       sum[ ok ] - csum  [ ok ]
                                       ftx[ ok ] - failrx[ ok ]
                                       rtx[ ok ] - rstrx [ ok ]
                                       itx[ ok ] - infirx[ ok ]
                                       ftx[ ok ] - failrx[ ok ] invert

This patch adds a 'grep' before 'sed' to fix this.

Fixes: b6e074e171bc ("selftests: mptcp: add infinite map testcase")
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
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>