powerpc/selftests: Use gettid() instead of getppid() for null_syscall

gettid() is 10% lighter than getppid(), use it for null_syscall selftest.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0ad62673d3e063f848e7c99d719bb966efd433e8.1622809833.git.christophe.leroy@csgroup.eu

tools/testing/selftests/powerpc/benchmarks/null_syscall.c

@@ -14,6 +14,7 @@
 #include <time.h>
 #include <sys/types.h>
 #include <sys/time.h>
+#include <sys/syscall.h>
 #include <signal.h>
 
 static volatile int soak_done;
@@ -121,7 +122,7 @@ static void do_null_syscall(unsigned long nr)
 	unsigned long i;
 
 	for (i = 0; i < nr; i++)
-		getppid();
+		syscall(__NR_gettid);
 }
 
 #define TIME(A, STR) \