selftests: mptcp: join: fix subflow_send_ack lookup

MPC backups tests will skip unexpected sometimes (For example, when
compiling kernel with an older version of gcc, such as gcc-8), since
static functions like mptcp_subflow_send_ack also be listed in
/proc/kallsyms, with a 't' in front of it, not 'T' ('T' is for a global
function):

 > grep "mptcp_subflow_send_ack" /proc/kallsyms

 0000000000000000 T __pfx___mptcp_subflow_send_ack
 0000000000000000 T __mptcp_subflow_send_ack
 0000000000000000 t __pfx_mptcp_subflow_send_ack
 0000000000000000 t mptcp_subflow_send_ack

In this case, mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"
will be false, MPC backups tests will skip. This is not what we expected.

The correct logic here should be: if mptcp_subflow_send_ack is not a
global function in /proc/kallsyms, do these MPC backups tests. So a 'T'
must be added in front of mptcp_subflow_send_ack.

Fixes: 632978f ("selftests: mptcp: join: skip MPC backups tests if not supported")
Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
Reviewed-by: Mat Martineau <martineau@kernel.org>

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

@@ -2776,7 +2776,7 @@ backup_tests()
 	fi
 
 	if reset "mpc backup" &&
-	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
+	   continue_if mptcp_lib_kallsyms_doesnt_have "T mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,backup
 		speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
@@ -2785,7 +2785,7 @@ backup_tests()
 	fi
 
 	if reset "mpc backup both sides" &&
-	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
+	   continue_if mptcp_lib_kallsyms_doesnt_have "T mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns1 10.0.1.1 flags subflow,backup
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow,backup
 		speed=slow \
@@ -2795,7 +2795,7 @@ backup_tests()
 	fi
 
 	if reset "mpc switch to backup" &&
-	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
+	   continue_if mptcp_lib_kallsyms_doesnt_have "T mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow
 		sflags=backup speed=slow \
 			run_tests $ns1 $ns2 10.0.1.1
@@ -2804,7 +2804,7 @@ backup_tests()
 	fi
 
 	if reset "mpc switch to backup both sides" &&
-	   continue_if mptcp_lib_kallsyms_doesnt_have "mptcp_subflow_send_ack$"; then
+	   continue_if mptcp_lib_kallsyms_doesnt_have "T mptcp_subflow_send_ack$"; then
 		pm_nl_add_endpoint $ns1 10.0.1.1 flags subflow
 		pm_nl_add_endpoint $ns2 10.0.1.2 flags subflow
 		sflags=backup speed=slow \