[POWERPC] Fix wraparound problem in smp-tbsync on 32-bit

The patch below fixes an arithmetic wrap-around issue on 32bit machines
using smp-tbsync. Without this patch a timebase value over
0x000000007fffffff will hang the boot process while bringing up
secondary CPUs.
Signed-off-by: NAdrian Cox <adrian@humboldt.co.uk>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/smp-tbsync.c

@@ -78,7 +78,7 @@ static int __devinit start_contest(int cmd, long offset, int num)
 {
 	int i, score=0;
 	u64 tb;
-	long mark;
+	u64 mark;
 
 	tbsync->cmd = cmd;