selftests: mptcp: join: fix local endp not being tracked

When running this mptcp_join.sh selftest on older kernel versions not
supporting local endpoints tracking, this test fails because 3 MP_JOIN
ACKs have been received, while only 2 were expected.

It is not clear why only 2 MP_JOIN ACKs were expected on old kernel
versions, while 3 MP_JOIN SYN and SYN+ACK were expected. When testing on
the v5.15.197 kernel, 3 MP_JOIN ACKs are seen, which is also what is
expected in the selftests included in this kernel version, see commit
f4480eaad489 ("selftests: mptcp: add missing join check").

Switch the expected MP_JOIN ACKs to 3. While at it, move this
chk_join_nr helper out of the special condition for older kernel
versions as it is now the same as with more recent ones. Also, invert
the condition to be more logical: what's expected on newer kernel
versions having such helper first.

Fixes: d4c81bbb8600 ("selftests: mptcp: join: support local endpoint being tracked or not")
Cc: [e-mail]
Reviewed-by: Mat Martineau <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 		ip netns exec $ns1 sysctl -q net.mptcp.add_addr_timeout=1
 		speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
+		chk_join_nr 3 3 3
 
 		# It is not directly linked to the commit introducing this
 		# symbol but for the parent one which is linked anyway.
-		if ! mptcp_lib_kallsyms_has "mptcp_pm_subflow_check_next$"; then
-			chk_join_nr 3 3 2
-			chk_add_nr 4 4
-		else
-			chk_join_nr 3 3 3
+		if mptcp_lib_kallsyms_has "mptcp_pm_subflow_check_next$"; then
 			# the server will not signal the address terminating
 			# the MPC subflow
 			chk_add_nr 3 3
+		else
+			chk_add_nr 4 4
 		fi
 	fi
 }